I haven't read the parma/aegis discussion as thoroughly, and I want to be careful about stepping on a landmine and hurting feelings. Plus at the same time offer perhaps a method for the plot to advance, as apparently this instance was important for that. I truly want to help and not make it look like I'm ganging up, taking sides or hurting feelings. If these suggestions don't work, just say they don't work.
- For the Parma/Aegis thing, I won't discuss the rules permutations and such but mainly go with story, internal consistency and history. The Aegis was created by the first Primus of Bonisagus based off the research of parma. If at the time it was created it had the ability to knock out the parma I would think Trianoma (who was still around) would have told him to go back and tinker with it to leave the parma intact, otherwise Tribunal meetings and friendly visits would be impossible. So, they go back to the drawing board and either tinkered with Aegis and Parma or both, and now the two work in harmony, and all future copies of Aegis as well as methods of teaching parma include these 'patches' so that this cannot be exploited. Since it's a Rego Vim affect anyway it's not as consistent that it can knock out or suppress parma IMO the way Perdo Vim can.
This is the method I use whenever someone thinks of some clever way to use parma as a weapon against its user, such as Muto Aquam on poison to turn it to water and have the magus drink it and die. Basically the magi over at Microsoft..er Bonisagus make a patch for it and send out the latest modifications to the parma to prevent that. The parma isn't smart or intuitive, but the creators of it are. The writers of the game have often stated that the Parma is not meant to hurt the user beyond the user's inability to shut it off if they are unconscious and thus resist helpful spells.
- For the plot device, some needed to be aware of a mentem affect that hit them and they resisted while others were unaware. A magic item with a high penetration and a target of Group should be able to do that and still affect the exact same people that were affected and fail against those it didn't affect. Blackthorn has the time, resources and talent to make one, and certainly has the motivation to do it, and since they have home turf advantage could pull it off without anyone noticing the attempt (beyond an affect hitting their parma, if that). Have that cause the same affect that was intended. Or one of their council knows the spell, can cast it without words or gestures and might have any number of non-AC bonuses that help penetrate, perhaps even has a magical discovery called 'Vow' similar to the Faerie Magic durations and targets, only this gives the benefit of an AC when casting on someone speaking an oath or vow; something like that
But something that's rules and story consistent can be used that doesn't turn so many conventions around.
I hope this was helpful, Sinmore would likely have been affected either way, and she already responded in character about it, and I'm ok with the results of whatever affect was placed on her and hope we can continue.