I'd go with the potion being reduced. It's not the wizard's magic that is up against the threshold in that scenario.

After rereading I see what you were trying to say. And I'd argue that Mab doesn't enforce the Accords at all, she just wrote them. She's as beholden to them as any other signer. The enforcers of the Accords are every other signer. Kinda like how the UN is governed by the body of countries and not a single one, if one country acts up they all smack them down.

I believe we have a Word of Jim stating that the reason the Accords exist is because Mab told everyone to sign them. Everyone did because she's Mab. So while the other signatories enforce them for the most part, it's Mab who is the driving force behind it all.

Remember during Small Favor, when Harry talked to Luccio on the phone about how it was in the Council's best interests to intervene in Marcone's kidnapping. He started off with the same line you're using, but Luccio was having none of it. It took Harry telling her that Mab was involved (and letting her draw the conclusion that Mab had threatened to revoke the guarantee of safe passage through the Ways) to convince her. The other signatories only enforce the Accords when it benefits them. Mab provides the background impetus to make them do it. Also, the entire point of Skin Game was Mab getting revenge on Nicodemus for breaking the Accords.

The Accords only work because Mab is one of the most feared and dangerous beings on the planet. If they didn't have that kind of backing, they likely would be like the UN; mostly impotent.

I'd probably go with a compel if you don't want the defender to just run away. Also, once it gets round to the defender's turn he's entitled to try and break the spell.

I'll put this in spoilers because it deals with side stories/comics that may not have been read by some, but here's what I remember of the Venatori Umbrorum.

Gaius Sextus is the eighty year old First Lord of Alera during the first five books. If there's an element he isn't a master of, nobody mentions it. He's also one of the two furycrafters to be able to manifest metal furies, which take the form of living armour that covers him from head to foot. He's also an astute politician, cunning and manipulative as Johnny Marcone, and married to Gaius Caria (daughter of High Lord Atticus Quentin) who has been poisoning for the best part of six years.

Araris Valarian is the best swordsman in Alera. He's also the second metalcrafter able to manifest a metal fury. He was a Singulare of Gaius Septimus, brother of Sir Miles Valarian (whom he pushed in front of a cart to stop him from fighting Aldrick ex Gladius) and (late in the series) husband of Isana. After Septimus died he branded himself with a slave's mark so he could protect Isana and Tavi without anyone recognising him, taking on the name Fade when he did so.

I think, given that it can be made by anyone with enough faith, that a Conviction based manoeuvre would be the best way to handle it. That said, holy water is a Catholic practice more than anything else, and Catholicism is heavily wrapped up in ritual so you could just as easily run it as a common ritual.

No specific reason. I believe we have a Word of Jim saying that it's possible, but that most wizards at the time of their death aren't exactly in the blessing frame of mind.

I'd say that a magic circle would be a Conviction Declaration or a Fate Point spent. It'd give you an aspect you could invoke for effect to just nullify certain creatures/spells/etc.