One of the characters in the campaign I'm running is a Witch of Thessaly, through Ex Miscelanea. Now these are, after my understanding, Infernalists, calling demons for their rituals.
Why did these ladies get an invitation to join the Order of Hermes, when they so blatantly break one of the rules of the Oath?
E.
IIRC (Heavy noble's parma there), Witches are non-hermetic, but some of them have joined the order, and renounced their former ways
According to the book, Trianoma herself was a member of the Thessaly tradition so I suspect that has given them considerably favourable bias amongst the traditionalists in the Order.
(quick consultation of rop:ti) For the most part they deal with necromancy and spirits, it seems... as do many other hermetic and non-hermetic traditions. Their infernal rituals need not be known as such, or practiced by all.
Quaesitors would probably be more suspicious of an ex misc witch of thessaly than a tremere necromancer...
But then as we know, Quesitors are all a bunch of corrupt status quo junkies who pander to those with the most political influence in the Order so their suspsicions are themselves suspect 
Well, from what I understand on the description in Infernal and Societas, the witches have Summoning which is used to summon demons, and each full moon they summon a specific sucubus for the Ritual of Pulling Down the Moon (or some similar name).
Luckily the resident Quaesitor 
E.
That's the infernal ones
I'm sure the Quaesitors do this too, but you seem to be conflating the pagan, Chthonic Magic using Ex Misc. Witches of Thessaly and the non-Hermetic, infernalist Daughters of Erictho. They are both of the same origin, but the Witches of Thessaly only practice Chthonic magic, and not necessarily infernal magic. Those who cross the line are in violation of the Code. Just worshipping or calling on chthonic gods doesn't make you an infernalist, it just makes it easier to start down that path.
The Drawing Dawn the Moon ceremony description states that there was a prior practice used by the Witches that is left undefined, and states that the Daughters of Erictho in Mythic Europe have a ritual they call the same name that is an Infernal Summoning ritual. A magic or faerie version of drawing down the moon is undefined. Sounds like a good article for the Sub Rosa.
Well, all Witches of Thessaly have Summoning, at least. And this may be Infernal, Faerie or Magic. But as for Commanding, that can only be Infernal, or so RoP: Infernal says.
I'm trying to find a solution to avoid having the magi declare some internal covenant wizard war. 
E.
Hermetic Witches of Thessaly don't have Commanding. See RoP: The Infernal, page 142, and HoH: Societates, page 109. Hermetic Witches of Thessaly start with Affinity with Vim, Summoning, and Painful Magic. Your Witch can be Infernal, but nothing in the RAW requires it.
But would the Witch then Summon creatures of Magic or Faerie power then? It says that the witches do have something to do with Faerie regios. Hmm.. what about the ghosts of the dead?
E.
Noble's Parma:
Summoning usually summon creatures of its type: Magical Summoning summons magical spirits... unless you're in a specific aura, when it summons creatures of the appropriate type. The only exception being divine auras, where it mostly summons demons.
Then, it could very well summon ghosts: Most ghosts are either magical or infernal
From RoP: Infernal P.114
While you may be dead on about your statement that summoning usually summons creatures of it's type, there is no game mechanic to enforce this. A summoner can summon any spirit that they can get themselves and AC for. (Although the appropriate realm lore is part of the summoning total and a sorcerer can add their hierarchy score to their total when summoning demons, so indeed infernal summoners will be better at infernal summoning.)
Of course
That's true. Commanding is essentially enslaving a spirit to the summoner's will, and neither Magic nor Faerie can do that, at least not to the same extent. If your saga's Thessalean witch has this Art, she should keep it secret because it can easily be recognized as infernal with InVi. Note, too, that Summoning of any flavor reads as infernal to Sense Holiness and Unholiness. If the Quaesitor possesses this Ability, the Thessalean will certainly have some 'splainin' to do.
Interesting dynamic for your troupe -- sounds like some fun opportunities for roleplaying. Of course, even if he identifies her infernal powers, it's not an immediate jump from "She has an infernal power" to "She deals with demons, and so has broken her Oath." I could see him putting her "on notice," making it clear that he is actively investigating her to determine if she has in fact dealt with demons to the peril of her soul and the souls of her sodales, so she had better be on her best behavior to prove her innocence.
If she is a diabolist, he'll probably catch her at it eventually (the demons will surely see to that) so the March is probably unavoidable. In that case, her part of the saga is trying to stave off her inevitable demise, so her player should be aware of that. If she isn't a diabolist (she fell in with a bad crowd, her family is diabolists but she's leaving that behind her, etc.) she needs to get the quaesitor on her side, and the best way to do that is to cooperate with him fully. If there is an outside threat (a demon, or her infernalist cousins perhaps), and he sees that she will use her powers against them to help him and the others in their covenant, perhaps he will not press charges against her.
By the way, a magus who casts DEO at everyone he meets will certainly get Hell's attention very quickly, as such a loose cannon is a dangerous threat to every demon in the region. It is not impossible for circumstances to lead to charges against him for endangering his sodales, if the demons decide the best way to take him out is by going after the others in his covenant. He's effectively declaring a full-scale war against the Infernal realm, and he will likely need every ally he can get. Perhaps there surfaces a plot to discredit him at Tribunal, and the Thessalean comes to his rescue?
Well, we did some changes to two characters in the group. First, the Witch of Thessaly no longer has Commanding. We changed that with Demon Blood. The story behind this is that her mother was a nasty evil Daughter of Erictho and her fathered by the Incubus/Succubus in the "Drawing down the Moon"-ritual.
With Demon Blood, the character is clearly tainted I would say, but her magic can be of the Magical Realm? It doesn't have to be False?
Also, not knowing everything about her past or powers, she set up a ward against demons.. from the inside. Now this lady has Infernal Might 7 from her Demon Blood. I ruled that she trapped herself in the ward, which had Ring duration. She can't affect the ward once cast if the has Infernal Might less than it's level, right?
Last question about Demon Blood. It says the character gets a power of level 30, which can be added to with Minor Virtue Demonic Power. When using this power, which costs might, will the player roll a die, or just use it like regular creatures with Might?
Also, the second character that was changed was the Tremere in the group. The players, new to Ars Magica, seemed to like playing Quaesitors, so he got moved to that house. He hadn't really played a Tremere up until now anyway
Eirik
He could have played a Tremere seconded to House Guernicus as a Hoplite, that would be one way to combine the two quite naturally.
Nah, he wanted to play a Quaesitor, not a Hoplite. We already have one of those.
E.
Note that according to the rules, a Witch of Thessaly (who is a Hermetic magus) cannot have Demonic Blood:
Furthermore:
The above quote from RoP:tI clears this one up as well. The character has to have powers aligned to the Infernal Realm.
Correct, although also impossible in a saga which strictly follows the rules as written.
Use it like regular creatures with Might. The character is not human.
Mark
Ah, I forgot about those rules. Seems like the character will change again.
Thing is, we have a With of Thessaly, and we need a suitable Major Virtue. It can't be Hermetic, she already has one of those, namely Chthonic Magic, but it would be great to also have Hermetic Sorcery. But these two can't be taken together, because they're both Major Hermetic.
Seems like Demon Blood is right out...
E.