IIRC, The errata ruling that said that you needed WC of sun duration also gave that to the Mercurians as part of their free WC. So they would still have 1/2 the level of their highest ritual of sun duration WC.
I haven't seen anything in the errata that was specific to Mercurians, although the copy I have with me dates from June (can't access it directly from my current location).
So, unless there was a recent addition, they only have access to (0.5*level)-10.
If you want to read it that way, no problem then. The magus invents a level-100 Aegis of the Hearth (may take a few seasons) and gets the same level WC. Now the magus and the apprentice can double the level fine since the don't have to cast it at that level. So we're back to the single magus with an apprentice.
But realize that this use of "know" is internally inconsistent. The spell doesn't say you have to cast it, just know it. How do you botch knowing it? Also, how do you rule the spell being used spontaneously?
I'm just going to suggest you reread the two things you quoted because what you quoted and what you got from it are drastically different. Just focus on those two points I quoted because those are what's way off.
Chris
Where does the 1/2 come from?
Chris
Do you routinely invent level 100 spells in your saga? 
But yes, the description of the spell is... strange. Rack that up to it being a remnant of old Mercurian rituals. Note also that the spell states (third paragraph) that only one die is rolled for the target spell, and that one botch die is added for each participant. So yes, it could be interpreted as not having to actually cast WC. This would be discussion for a different thread, though, as it's peripheral to the current discussion, IMHO.
Yep, I misread there. Sorry again.
My overall point remain, I believe. It's a lot of efforts for a relatively small vis income.
It assumes this is your highest-level ritual that you are trying to cast using WC. Since you automatically learn WC at a level equal to the level of the ritual, and you must double the level of the target spell with WC, you are basically providing half of it already. (My position is that this is minus those 10 levels that were used to make WC at D:Sun.)
No, but pulling off a level-50 spell with someone who's got Arts in the low teens isn't so uncommon. So I figure by the time the TeFo total is closing in on 60 or so it shouldn't be that hard. I did pull off a lab total around 200 for one season with a maga about 10 years out of gauntlet, and that was prior to Covenants. She could do 120s or so regularly. So this wouldn't be so bad for someone whose lifeblood is in Aegis of the Hearth.
Which still brings up the question of how you botch it.
Hmmm... I'm not sure I buy that. So this magus is already a Mercurian and so already built toward ritual magic. As you mention, Artes Liberales, Philosophiae, Penetration, and Stamina have lots of other benefits. The Talisman does as well. If you're a Rego + Vim specialist, you're working on those anyway. So, assuming you're a ReVi specialist Mercurian, how much extra effort do you put in? You invent a few Aegis of the Hearth rituals (and get WC for free). You master Aegis of the Hearth (which also lets you design the others more easily - similar spell bonus can grow). Let's say you put 20 seasons into Aegis of the Hearth and mastering Aegis of the Hearth; everything else is aligned to anything else you do and so isn't an investment here. What is your return on those 20 seasons? Let's say you get four (one per equinox/solstice) covenants (your own included) in the entire Order who essentially pay you 2 pawns of vis per year to cast this ritual. This takes essentially no time from your laboratory since it's only one day per season and doesn't even cost you long-term fatigue. So after those 20 seasons of investment you get 8 pawns/year on top of whatever you do in your seasons, and your covenant saves a little too. Forty years later those 20 seasons have netted you an average of 16 pawns apiece. 16 pawns per season sounds pretty good to me. I think the issue is that there is a lot of up front work. However, it can be very worthwhile.
Oh, got it. Half of double the level, not half the level. I don't think it was very clear as written.
Chris
I still think there's a lot of assumptions there, on the amount of time needed to get there, on the market for a specialist casting a level 60 Aegis, etc.
It could happens, but by the time the magus reaches the point where he has those stats, he'll be mature and may want to do something else.
But let's come back to the original poster's questions. 
Possibly, but for the most part these would be few. Spring covenant with no one knowing Aegis (who would just want a level 20). Perhaps a well-established covenant threatened by strong supernatural creatures.
No token, for sure.
As stated previously, most customers would not want Aegis done on days other than the solstices or equinoxes, as this would leave them vulnerable. So you only have 4 days in the year where customers want this service. So quick travel is mandatory.
What he would do the rest of the year depends on his interests. Probably something to do with rituals, Rego and/or Vim, as these need to be his specialties.
Cautious Sorcerer is very valuable for someone who casts rituals. A relevant Minor Magical Focus makes it easier to get into a position where your services are valuable. Good Bargain skill, raw vis trading (he's not neccessarily paid in Vim). Free Study, Inventive Genius, Magical Memory, Personal Vis Source, Mastered Spells, Puissant Art may all be interesting in their own way for this specialist.
Provided that the magus has access to a mean of quick travel, his lifestyle wouldn't be impacted much by this. So his actual personality and interests can be wide-ranging.
For example, I currently have a Bonisagus Rego Terram specialist with Mercurian Magic. He will be doing original research into Hermes Portal to develop something more flexible and useful. He plans on offering his services as a ritual-caster (though mostly in Rego and/or Terram). Spells like Conjuring the Mystic Tower (and variants), laboratory creation, covenant-building spells, etc. But that's just a way to fund his research.
That's right - but see below for the problem.
So the expert can cast a maximum of four Aegides per year - provided he can find for each solstice and equinox a covenant, where the Aegis expires at just that time and the magi are willing to contract him.
This is very campaign-dependent - but does not look like a full time occupation.
Cheers
I think you missed the point that it was an example. That point is that it is a reasonably fixed investment that will continue to yield future returns at essentially no cost. Eventually this will be worthwhile.
Let's say you only do this for your own covenant and it takes 40 seasons. 40 years later you have gained 80 pawns and have saved you own covenant 160 pawns. Let's say you get 10% of your covenant surplus. So with no foreign buyers those 40 seasons are already worth 2.4 pawns each. Let's now say you find one covenant (spring figuring the can get a level-30 for the price of a 25, barely over the price of a 20) per year willing to pay you 2 pawns for a level-30 ritual with good penetration. Now your seaons have earned you 4.4 pawns each, and that is if you spend 40 seaons on stuff you might not have and only find one buyer. People seem willing to distill vis for returns worse than this.
Chris
I do not follow. Sure, the contract is necessary. But the seasonal part is not a problem except for the first time, and you could offer a 1-pawn discount on that one. But 4 maximum? They only take 3 hours and aren't very tiring.
Chris
Speculating that Magi would never agree to go without an Aegis for any period of time, so the only way the magus who does this to make a living is to cast it on the Solstice, as the previous one expires... These would be some awfully paranoid magi... And of course, those who want such high levels of service should be willing to pay extra for it, or learn the darned ritual themselves. 
That's what the contractor competes with: so far nearly every covenant had a member cast its Aegis at exactly the right time.
just imagine: "Listen, I can give you an Aegis for cheap, very cheap - but you have to stay for three days without any Aegis before ..."
Cheers
Maybe, maybe the contractor got in on the ground floor and nabbed a sweet deal, and the magi pursued their own interests that were not Rego or Vim... Such characters do exist.
And so, liked a frog placed in tepid water, the price starts going up slowly...or the day changes ("Sorry, guys, I'm too sick to cast the Aegis today), and the covenant has no real enemies... Or whatever.
Actually, I completely get that. What I'm driving at is that it will be decades before the magus gets to a point (stat-wise) where he can do this. And that the yield from that investment depends on how long it goes on. The longer it takes him to become powerful enough to becaome that kind of specialist, the fewer years he will have to make it worth the investment.
How old do you think a specialist like that would be? How old will he live and still want to perform those services? The answers to that depend on the saga, so it really is up to the original poster to come up with answers for this. (Might also apply to you, if you plan on having such a specialist in one of your sagas.)
I'm just saying that assumptions need to be made, to make that kind of specialist realistic in a specific saga. Apart from the trust issue, there is the power-level issue, and the willingness to keep doing this every year, for decades. And the magus must have a personality suited to this kind of work.
Then there are story implications. What happens if the specialist gets delayed for one of his customers (say because of a Twilight episode)? Or was not as trusttworthy as his customers believed? Would he have space on his schedule for a new covenant? Is there a good enough customer base to support him? Is there competition?
Quite possible indeed, with enough dumbbells around.
And then the contractor and his trade get known in the Tribunal: so knowledge of his schedule around equinoxes and solstices becomes a valuable commodity, too. Who pays then to keep it out of the wrong minds? And how much? And even if some faerie does not find it out: it might just spy and try around the solstice / equinox anyway - and perhaps slip in if it can.
There is a reason why Aegides are usually smoothly following each other: a single compromising of them can have consequences at any time in the future. I can't imagine the magus who would take that risk easily to save a few pawns of vis - but I am notoriously incapable to imagine certain dumb reasonings.
Cheers
Let's not call it dumb, and instead call it invested in other things. People, in all walks of life take actions and based on certain assumptions,and only when things get turned upside down do they question their assumptions.
Yes, I assumed that. I still don't follow for two reasons. The first is that seems to give a maximum of 8/year, assuming they're all level-60. If they're of lower levels you might be able to squeeze in a little more. Most likely based on average locations you might be limited to 6/year. You know the ending of Year is based on sunrise, right? Second, even that means you go without an Aegis for a very brief span of time. If magi really don't want any time without an Aegis, then they need to cast the Aegis every 9 months on average. So if the assumption is what you're speculating, then the magus can do roughly 4 per day all year long and is limited to a total of roughly one Aegis per magus in the Order.
Chris
Sounds very much like players gaming the SG to me - with a reasoning like "naah, that's not the stories he likes, so lets not invest in being prepared just for these".
In Mythic Europe at large, a single case of magical or faerie pests moving into a covenant during winter solstice while the Aegis is down would kill the business proposal of the contractor for good.
Note that:
Recasting the Aegis will not oust the pests afterwards.
Even detecting them might be a problem.
Spells affecting them in all the covenant at once would be either Range: Boundary, hence Rituals taking time to invent and vis to cast (yes, just that time and vis promised to be saved) - or use huge, unreliable Circle Targets.
And scouring the covenant room by room might never be completed, but will most likely just chase them around.
How's this to get a feeling for the situation: youtube.com/watch?v=84LD5ektDkQ ?
What I can envision is a contractor bolstering an existing Aegis for special occasions like impending Wizards' Wars, conflicts with powerful beings flaring up, or important meetings not to be disturbed or scried upon. And in such a situation a covenant might well pay a premium to have a specialist cast a really powerful Aegis reliably just for the remainder of the year.
Cheers
And if it is the SG proposing it, are they gaming the SG?
Also, in a setting where this is done, as described in the OP, it's an NPC, the SG is proposing it, he can set a lot of things and make a lot of the assumptions about this world that he likes. If the troupe likes it think it's will tell interesting stories, all the better. In that vein, it might be better to treat Year Duration as an actual calendar year.
There are many ways to skin a cat. People want to pursue the things they are interested some are very well not interested in ever learning the Aegis. Would they hire someone to do this on their behalf, if they trusted him? Would others do it? Even going by RAW, he could do 4 rituals per solstice or equinox, on average, 16 Aegides a year. Presume he has means to Leap to each location, get's busy and just does it, and leaps away. He might expect payment up front, or might come back and collect it the next day. Is it possible? Yes. Is it likely? Again, it depends on some assumptions used to build the saga...
Where's that coming from?
An Aegis expires (ArM5 p. 112) at "sunrise on the fourth equinox or solstice after its casting". So the new Aegis needs to be cast after the astronomical equinox / solstice - because otherwise it will not last a year. And it needs to be cast before sunrise, because otherwise there is a gap between Aegides.
Given that the times of astronomical equinox / solstice and of the sunrise following it at a certain place are not correlated, there can be a split-second or a day in between. So you might have to be verrry precise with your Aegis Ritual at one occasion, and cannot accomodate more than one customer then. While you can perform several Rituals in sequence easily at another - provided you ReCo around and convince all of your clients knowing some astronomy that when there is not much time to complete the Aegis next time you of course will come to them first.
Unfortunately most magi know some Artes Liberales, and hence understand equinoxes and solstices sufficiently to ask unpleasant questions.
Cheers