My thought would be to make it MuCo with an aq requisite then base the spell on the guidelines for creating poison: the MuCo would be base 10 with target part 9because only part of the body is being transformed- teeth to fangs and saliva to poison), changing a liquid into a poison doing a heavy wound is base 4, so raising the base to 15 should suffice on the aq requisite... causing a fatal wound is base 10 and should probably require an extra magnitude.
Oh, yes, changing the saliva to poison is a great idea too!
I agree that an argument might very well be made that a corpus requisite is needed, but I don't think it is, as the saliva is no longer in the body. This'd make it a straight MuAq spell.
I agree that a MuAq with no Co requisite sounds right for changing saliva to poison.
Just make sure you're immune to that poison first. 
Part of why I included the Co is to contain the transformed saliva in the fangs... which double as a delivery system.
So, it could be MuAq(Co), Base 4 to "change a liquid into poison causing a Heavy Wound", R: Touch +1, D: Momentary, T: Part +1, EF 9 +2, for a final level of 20. Which is still not doable for her (unless this is something that she would have access to a Laboratory Text for, it being all snakey and poisony and so fitting for her Tradition).
And I think the Corpus requisite is appropriate for the fluff text to change her teeth into fangs just long enough to deliver the bite, mainly because it's more visually awesome. I can see her casting her spell (speaking Parseltongue and doing the sinuous gestures), then grabbing her victim as her canines elongate into fangs and biting.
So, there are two ways, it seems, to do this poison bit thingie of Guiverna's.
The MuAq above, and the PeCo in her Development thread. This could very well be a "more than one way to skin a cat" scenario, but I can where both ways are effective.
So: straw poll time. Which one do you think is better under the circumstances. Not really "which one is more cost-effective", but more "which one comes closer to doing what it's intended to do?
My own thought is that the CrAq(Co) spell is a bit more appropriate to the theme you're going for.
That's what I'm leaning towards, too.
I just had an idea. Maybe the fangs should be a different spell. Granting In+0, Atk +3, Def +1, Dmg +1. Or if you are gonna add the requisite, take +1 fang damage fr the effort.
As for your struggles to hit the right lab totals, I say make the character five years older. Or put together a five year plan. Or customize your lab for spells.
I personally like MuCo(Aq) better- for one thing depending on duration it can affect multiple victims, which seems more in line with the intent- to make bodily weapons more than just to kill/incapacitate a person.
Also, as someone noted, it can be used on animals, which a PeCo spell would not.
Ok, tell you what?
Here's her sigil: When casting her spells, either Guiverna or her target briefly takes serpentine features (fangs, forked tongue, eyes, make snake sounds).
Now, for each and every one of your spells, describe how this manifests^^
So, for exemple, if doing a "Kiss of Death" variant, instead of "Kiss", write "bite".
Oh, yes, this is very cool, and she could totally do that. Such a low-level spell would also have a good penetration.
That was me 
PeCo, if it penetrates, causes a wound. No soak or else, bim!
Poison, if it penetrates and isn't resisted, also causes a wound which is not soaked... But works on anything, from humans to giants.
I realize, I hate it when I say "you can't do this" to someone, but I also don't like when I'm saying "I'm telling you, you can do this! Promise!" 
I've been statting Bernat's armaments. Part of his concept is that he carries a set of throwing knives - that is, not just ordinary kitchen knives, but proper weapons balanced for throwing. They can be up to Standard cost from his background, and it seems likely they might be, as he would presumably have been armed by the Bishop of Urgell while he was employed there.
However, the RAW only gives stats for "Knife: Any sharp knife of 6 inches or lesser length... they are everyday tools", which isn't really the same thing. Would a set designed specifically to be missile weapons (and potentially of Standard expense) be allowed better stats? If so, what? I had thought of maybe modelling them on the Throwing Axe / Hatchet, but that has Load 1, and it's pretty critical that Bernat could carry a small set - six, say - without it hindering his stealth.
I would think so, GURPs would think so, Ars Magica does not.
You're leaving an important bit from your quote, which changes the meaning from "these are the stats of ordinary knives" to "everyone has knives".
"Any sharp knife of 6 inches or lesser length. Knives are even more common than daggers; they are everyday tools."
Ars Magica's emphasis isn't on combat, thus the simplified stats. One could make similar arguments for all kind of weapons.
Are your knives under 6 inches of length? Then they use these stats, unless of greater quality.
=> If you really want better knives, I'd suggest taking Superior throwing knives, giving +1 to attack for 2qp each. This means that each pawn of vis gives you 2 such knives.
Eh, that would be an improvement. How, as a companion, do I acquire q.p.v. equivalents?
It's easy and cheap!
Before game, 1XP = 1qp, so if you want, say, 5 such knives, it only costs 10xp.
I might consider that. It'd mean adjusting my starting scores, but it could be worth it. He does have a number of 1s that could be dropped.
So, when Guiverna invented her poison saliva spell in Summer of 1235, she experimented. And she experimented HardCore (+3 Exceptional Risk), since otherwise was 7 points shy of being able to invent it in one season. She pulled it off. And then came the Extraordinary Results roll. Die roll of 6 + Exceptional Risk 3 = 9: Special or Story Event: "Either some effect not covered elsewhere occurs, or, at the storyguide’s option, an event unfolds as a result of your work which involves the entire covenant." This would (probably) have been after she came to Bellaquin.
Any ideas?
Also, she has invented (from scratch) two MuCo spells in the same season. One is Baring the Serpent's Fangs, which changes her teeth (and probably her jaw, somewhat) into sharp fangs. Base 2 to change the target and give them a minor ability (in this case, the fangs which do +1 damage), +1 Diameter, +1 Part. The other is Donning the Serpent's Scales, which is effectively a watered-down version of Gift of the Bear's Fortitude, in that it grant's +1 to Soak. Base 5, +1 Diameter. This spell changes her skin into snake-like scales for the duration of the spell. I know any modifiers to the lab total have to apply to all spells invented in that season. Would both of these qualify for her Potent Magic: Snakes?
How about instead of inventing a spell you invent a spell spirit instead?