You forgot to remove the part about the spell being harnessed. 
MuCo spells are harnessed for all mages, because it has been integrated into Hermetic Magic for MuCo, says HoH:TL, p. 97 (and the spell descriptions in the core book say the same). I have always read this as referring to shapechanging magic only, therefore the mastery for The Man Who didn't bleed. If it really applies to all MuCo spells, I have 5xp left, which I could put into Durenmar Lore.
Not all MuCo spells, and it is not true harnessing. From my reading, the integration of harnessing into MuCo spells is limited to shapeshifting spells, provided that a piece of clothing is used as a way to end the spell. Without the use of the item of clothing (such as a cloak of feathers), the target of the spell cannot end the spell at will. The MuCo guidelines are clear on that (ArM5 p.132):
Certain shapechanging spells allow the target to change back by concentrating. While concentrating, you symbolically remove some item to break the spell, such as a wolfskin cloak or some water in your hands. These foci are essential if you are to be able to break the spell in this way. If you cast the spell without them, you must either dispel the magic with Perdo Vim, or wait for the duration to expire.
True harnessed spells don't need the item of clothing, and can even be ended at a distance (see HoH:TL p.101 under "Mutantum Magic"). So the version of the spell that your magus uses is not a harnessed spell, just a regular one. A magus without the Mutantum Magic or Harnessed Magic virtue cannot learn a true Harnessed spell (see HoH:TL p.101 under "Mutantum Spells").
Note that the integration in MuCo spells also include an element of Tethering, since control of the spell is tied to the item of clothing, allowing the target can end the spell by removing the item.
Ah, okay then I'll shift mastery to the shapeshifting spell
How does inventing spells pg work? Can I assume that a lab text exists for core book spells and what about spells from HoH? Or is there some kind of rule for getting your hands on such a text (like a fixed vis cost of 1p per 5 lvls?)?
No strict rule, just the narrative approach (as detailed here) and common sense. If the narrative has your magus making friends with a Bjornaer magus and trading to get access to Voic of the Bjornaer, for example, that is fine. Want to visit Durenmar to search the Great Library for a variant spell? Certainly possible but probably have to ask a few years in advance, pay the visitor's fee and spend a season locating the desired lab text. Of course, as a Mercere magus you can rely on the redcap network and House library (which is probably specialized in a few limited areas), but you shouldn't abuse it.
If it's in one of the official published (see here for a hierarchy of the books), it is probably possible to find a lab text somewhere. But it should probably require some time to locate it, and may require some trading.
If in doubt, just ask. 
Year 1, winter:
Placidus arrived at Fengheld before the first snow fell. He had covered the distance unaccompanied, enjoying the air buffeting his wings as he looked down on mostly forested land with too many plumes of smoke rising from below and too many hamlets nibbling at its edges.
It was early afternoon when he landed outside the Aegis to walk the last few miles. The Harz was not that different from the Schwarzwald, forested mountain ranges both, so he felt quite comfortable. That first impression melted away when he saw the size of the covenant: It certainly wasn’t the forested idyll he had hoped for. But winter was coming, so he couldn’t be choosy: He needed some warm place to weather the storm.
He touched the three golden eggs in his pocket: They had a parting gift from Xavier, enough vis to set him up as a Peregrinator for a few years. He wasn’t sure what to expect. He had a letter of invitation by Horst, not surprising, since Fengheld would love to have their own Gifted Mercere. And maybe it would keep his mother of his back for a while.
He was taken to a room to freshen up, before being presented to the magi: Eight of them had left their labs and were staring at him curiously over dinner. There was some smalltalk, but they got down to business quickly. Placidus presented his Gift and after some haggling, the terms of his stay were set: He would be a guest for the next seven years, and allowed to access the Peregrinator section of the library, containing junior spells and primers, but he wouldn’t be allowed to copy any of the texts nor be given access to the Master library. That was somewhat disappointing, but they had made him an offer that sweetened the deal. He could claim a bed at the main house and any chapter house for up to a season, and would be allowed to use a lab already set up at the Lusatia Chapter house two hours of flight to the east. They also gave him back his vis as a gesture of good faith, but told him they’d expect him to spend one season per year to further the interests of Fengheld.
Around noon the next day, after a long talk with Horst about Mercere business, he checked out two slim volumes on handling vis at the library and made his way towards Lusatia.
The site of the chapter turned out an idyllic if swampy site on a hill safely above the river populated by two families of servants and a moody Bjornaer maga who only grudgingly accepted his presence after he showed her the letter signed by the Fengheld council. After a short exploratory flight around the covenant, Placidus withdrew into his lab and started learning the spells detailed in the books he had borrowed.
Fengheld Peregrinator Mechanics:
- a season of work (in the form of adventure) is expected per year
- the junior library contains primers on all arts Lvl5Q15, all corebook spells up to lvl 10 (and no others), and some tractatus on abilities (none on arts, nor mastery).
- he starts with a one time Gift of 3p Animal vis
Seasonal mechanics:
He invents Scales of the magical weight and Sense the Nature of Vis using a lab text, which nets him 2xp in Intellego.
Yearly benchmark:
3/3 p of vis gained, 12/45xp gained (counting a spell level as 1xp), 0/1 Warping point gained
Note: Is this how it is done?
You realize this means Placidus needs to include a Terram requisite when casting King of the Air? That isn't too much of a problem when he casts the spell in a decent magical aura (he'll often lose Fatigue and sometimes fail to cast the spell). I realized this when I started reading the narrative with his travel to Fengheld and Lusatia. Book covers usually contain wood and/or metal as well.
It is indeed! 
The access you are given to the library is certainly conservative. You may be able to negociate an occasional access to a better summa, or more advanced lab text, in exchange for additional services.
In the yearly benchmark, you can count a season of lab work as a flat 10 xp instead of calculating it by spell level.
I guess he could use ceremonial casting to get an additional +2, and I do need to pick up some philosophy. I thought the armor was bone and leather only.
Lords of Men describes reinforced leather armor like this:
Reinforced leather or padded material strengthened by many small, hard, places or rings. These reinforcements are usually metal, but horn, bone and whalebone are sometimes used.
You are right about transporting the books though (and I might possibly need herbam as well, depending on the ink): Reading a primer will give Placidus 5 in each eventually.
Indeed, horn and bone could have been used instead of metal for that specific armor. Still, casting requisites will often be needed. I doubt Placidus would want to travel without even an eating knife, or maybe a small hatchet for chopping firewood. Rope is made of hemp. Most food is animal, but not bread. Wine or ale is Aquam (most people didn't drink plain water in inhabited areas, as it could make you sick due to contamination by e.coli).
Casting requisites are not a big deal for the narrative, just something to keep in mind, as it can make carrying something back home an issue (such as raw vis in various forms) if you can't cast King of the Air in a good magical aura or if Placidus is in a hurry. Might make a nice element to integrate into the narrative at some point, in fact!
Year 1, Spring
From Placidus diary:
St. Agnes’ Day, at Lusatia
I was awoken by the sun today! Winter is finally coming to an end. It has been in the air for days and I have seen Galanthus white heads not far from the covenant. I’ve packed my books to return them. I’ve decided to walk to Fengheld. I want to get a feeling for the land, and I am sure I am going to find someone or something interesting along the way.
St. Patrick’s Day, at Fengheld
I’ve returned the books. Horst has asked me to investigate tales of a White Stag devouring one of the covenant’s herbam vis sources. A Fengheld hunter named Stefan Blutpfeil and four grogs are coming with me to show me where they have last seen him. That Stefan is an unsympathetic person boasting how many magic beasts he has slain. Some of his tales are obvious fabrications, but he has shown me a chain around his neck with a baby drake’s tooth on it, and I am convinced he’s quite a butcher.
Servatius Day, back at Fengheld
Stefan Blutpfeil is dead, killed by the stag, a fitting end. When I tried to talk to the stag, that fool shot an arrow at it and was trampled to death. I was slightly wounded by one of our own grogs, when I stepped between them and the stag as they wanted to revenge themselves for Blutpfeil’s death. I managed to calm both the stag and the grogs, and was magically healed by the stag’s tongue. It turned out that Blutpfeil had been hunting the stag’s fawns, believing them to be magical beings as well and hoping to steal their vis for himself. There is now peace between Fengheld and that glorious animal. The stag has returned to the deep woods and the grogs have been demoted to night guard duty. I shall return to Lusatia with a copy of the Rego Primer, to study in the cool shade of the lab during the summer months.
Seasonal mechanics:
Adventure, 7xp Magic Lore from 1 to 1(7), Reputation: Protector of the Wild Places 0(3)
3/3 p of vis gained, 22/45xp gained, 1/1 Warping point gained
Year 1, summer:
Placidus returned to an empty chapter house, except for the servants, who told him that there was nothing unusual about Odorpes absence: She spent much of her life as her bear heartbeast. He read the Rego Primer. He also helped the servants to make a smokeless fire with a simple, spontaneously cast ring spell, which made them distrust him less. Still he decided, he needed to improve his Parma soon to cover them for a more relaxed coexistence.
Smokeless Ring of Fire (based on Apprentices, p. 47, spontaneously cast)
The fire don’t smoke no more (as one of the servants’ kids put it).
(base 1, +1 circle, +2 Ring)
Seasonal mechanics:
Book study: +15xp Rego, which raises Rego from 0 to 5
3/3 p of vis gained, 37/45xp gained, 1/1 Warping point gained
Year 1, Autumn:
One morning, one of the servants lingered. When he asked her for the reason, she eventually said that Odorpes had been missing for longer than expected. Asking around, the young Magus tracked her to Pomerania, where he found her, badly wounded, surrounded by werewolves that seemed rather more intelligent than the ones he had seen so far. Not much of a fighter, he stayed on a tree and pelted the wolves with stones until they retreated. He helped Odorpes retreat to a cave, a more defensible location, and then flew to Crintera for help. Although he had expected someone to come for her, he was surprised by how fiercely the Bjornaer reacted to his news and that, once Odorpes was safe, even taught him a few parma tricks.
Seasonal mechanics:
Adventure 3xp Magic Lore from 1(7) to 2, 5xp parma from 1 to 1(5)
3/3 p of vis gained, 45/45xp gained, 1/1 Warping point gained
You can boost the xp obtained from the Spring and Summer seasons a little bit (to 10 xp), since Summer was spent reading a low-level primer. Not all xp are treated equally and I tend to allow a bit more when the seasons are spent on the basics (like reading a primer). The 45 xp per year are an average and a guideline, not necessarily a strict number.
Edit: SummerSpring is intended to be the season of service bargained with Fengheld for the first year, correct?
I don't actually need those xp atm. And the service season was spring (does it really matter anyway?)
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1195, winter
On the way back from Crintera, Placidus once more visits Fengheld, after fetching the Rego Primer at Lusatia. The grogs ask him to provide them some warmth and light for the winter, so he asks the librarian to borrow the Ignem Primer for the cold months. He has a run-in with a Jerbiton Journeyman named Imanitos Mendax who expresses a desire for the same book. Placidus loses a Muto Mentem certamen to the glib Jerbiton. Imanitos Mendax casts a humiliating spell on Placidus, whose shame is turned into lust for Brunhild, the ugliest maid at Fengheld. When Placidus wakes up beside her the morning after, he apologizes to the maid and promises himself to one day make that Jerbiton pay for his cruel joke. A spell of cold weather keeps him from returning to Lusatia, so he reads the Terram Primer at Fengheld. There is talk of a grand tribunal all winter , and the young magus decides to attent to lobby for the cause of the Weissdorngilde.
Seasonal mechanics
Reading a primer on Terram: 15xp, raises Terram from 0 to 5
0/3 p of vis gained, 15/45xp gained, 1/1 Warping point gained
1195, spring
Learning that the Grand Tribunal won’t start until May, Placidus decides to walk most of the way to Durenmar. A few days after leaving, he comes across a magical fox trying to catch a magical raven. The raven promises Placidus to foretell his future if he saves his life. The magus, who is curious manages to scare off the fox. The raven, who goes by the name of Doom, warns him that attending Grand Tribunal could be dangerous. At first, Placidus doesn’t believe him, but the bird keeps following him around, and eventually dissuades him from going to the tribunal. Instead, they slowly become friends and explore the forests and even find some animal vis in the form of a discarded snake skin.
Seasonal mechanics:
Adventure: magic lore 5xp, raises skill from 2 to 2(5); familiar candidate found (Doom, a raven of virtue), 3p animal vis found
3/3 p of vis gained, 20/45xp gained, 1/1 Warping point gained
1195, surely. No TARDIS in this saga!
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Placidus familiar binding lab total is 23
In 7 + An 8 + Int 2 + MT 3 + Aura 3 = 23
If he were allowed to use a lab at Fengheld main, he would get a total of 25, rather than 23. Is there a way he can rent a lab (he can pay up to 1-3p of An vis as rent)
Cost: 3p to forge the link
Cords
Bronze 1
Silver 0 or 1
Gold Cord 2
Are you using the rules that familiar need to spend vis so they can get xp in MT?
Does being bound as a familiar count as MT exposure for the familiar?
That possibility was not available to the other players, so for the sake of fairness I have to say no. Or rather, the lab being rented would have some characteristic that negates the effect of the higher aura.
Rules from RoP:M pp51-52 apply to familiars. So Might is substracted from Advancement Total, but 1 pawn of vis can nullify 2 from the substraction.
That makes exposure pretty much worthless as a source of advancement for most familiars, but being bound as a familiar might count as exposure, provided the magic being is already intelligent (Int score instead of Cunning) and some way to communicate with the magus.
Can I then get the Redcap Horst ex Mercere of Fengheld to teach the familiar Magic theory 1 (lab assistant) for a season - this would be a once only thing, not something done repeatedly.
Horst is a famed author on Magic theory, so I assume, he has Good Teacher and quite a bit of teaching ability:
Single student 6 + Com 3 + good teacher 3 + teach 2 would be 14, enough to overcome its might of 9.
Placidus could offer Horst 3p of vis and taking over his normal duties for a season, or he could use his vis and season to create a lesser item for him, like a doublet of impenetrable silk.
Doublet of Impenetrable Silk (lvl 10 item); 1p of vis
Cloak of the New Man (lvl15): cost 2 vis
Allows to assume other human forms
Base 3, +1 touch, +2 concentration, +5 24 uses/ day
Ring of Ennobled Presence (lvl 10): cost 1vis
(like aura of Ennobled presence, 1 use/day)
Or three Rings granting Eyes of the cat (3x lvl5): cost: 3 vis
It is probably possible to hire a teacher for Doom. Two things to take into account.
First, Horst at this time is 24 years younger than in Guardians of the Forest, so only 38. Thus, his fame as an author may not be as well established nor his knowledge as great. That works as both an advantage and a disadvantage for you. It is an advantage because he might charge less for a season of his time. It is also a disadvantage because as a younger redcap he probably has fewer available seasons to teach. In fact, he may not even be a member of Fengheld at that time!
Second, you made a small mistake in the teaching formula, which is Teacher’s Communication + Teaching + 3 + bonus (you forgot the +3). So someone with Teching 1, Com +3 and the Good Teacher virtue would have a single-student Source Quality of 17.
Now, if a teacher with a single-student Source Quality of 14 is in Magic Theory is good enough for you, I think you will be able to find one at Fengheld. This requires only a Com + Teaching + virtue total of 5. This could be Horst (if he is indeed living at Fengheld at the time), a younger magus (perhaps a peregrinator like yourself) or even a late-stage apprentice with good teaching skills. That would probably cost the equivalent of 2 pawns of vis. An enchanted item is less likely, since it is worth the cost in vis plus the value of a season. But if you want to pay with an enchanted item that is fine with me, you may even end up gaining a pawn of two for the work (and will have a lab text for future use).
A Quality 17 teacher would cost much more, since fewer will be available and they will be more experienced. That would be worth 3 to 5 pawns, depending on the value of that person's time.
BTW, I just created Placidus' stats topic.