Sorry late reply and I couldn't read all 25 pages of posts between yours and now, so apologies if my reply is already somewhat covered elsewhere.
You are right that she technically can leverage that, but that is not her personality. She is all about "doing the right thing" because there's a touch of being a naive character in her. So when Tybalt was being an ass on Rooster, she had no problem defending MC. When MC trespassed, she scolded MC. But she's willing to defend MC because Tybalt banning MC from visiting is not right. However, in this case, it's not "right vs wrong", MC is in the wrong too. So it's "wrong vs wrong" which makes it difficult for her to defend MC.
This is amplified by the act itself. When MC trespassed to visit Jill, it was also "wrong vs wrong", but the magnitude is much smaller as there's no harm done. In the party's case, it's no longer an innocent mistake. It ruined the party which affected all the people attending the party, including the guests, and potentially causing property damage. The scale of the problem is much bigger.
So now we're at "wrong vs wrong" and "it's a bigger wrong" (plus violent conduct if MC punched Tybalt) and it's "affecting multiple people". To pull something like "I'm not going to talk to you if you sue my friend that trespassed and ruined people's night and punched you" is basically a very cheap move that wouldn't suit Jill's character at all.
This is where it showed about the character. Jill being meek is part of her character. Her countering doesn't fit her meek character. You can't project yourself into Jill in a "what would I have done if I were in her shoes? Oh, I would've countered and threatened not to talk to Tybalt anymore instead" because she has a different characteristic than you. Different characters simply have different ways on how they behave.
Whether you like Jill as a character or not is fine, but I think she behaves consistently as a character.
I agree with the first half, but not the second. Yes, this situation is too serious and the MC too culpable for Jill to simply get him out of it. But I disagree that Jill is meek. She's reserved, but I don't see why you think she's meek. Jill has consistently stood up for what she thinks is right. Even if, as you say, the MC is in the wrong here, that doesn't make what Tybalt proposes acceptable.
Frankly, I think the Tybalt situation was presented very poorly. DPC tried to justify Jill's acquiescence by having Tybalt play good cop ("Oh, the other preps want to sue him, but I'll stop them.") while simultaneously leveling direct demands at her ("Stop seeing him and be my girlfriend or else!"). You can't do both. If Tybalt wants to blame the other preps, he needs to make no demands and just count on guilt tripping Jill into spending time with him. If he wants to compel her actions, he needs to admit up front that he's the one pulling the strings.
Because as presented, it's a transparently obvious ploy and Jill looks stupid for not putting two and two together. If Tybalt isn't the one forcing the issue, why does she need to placate him instead of talking to the other preps directly? And if Tybalt IS the one forcing the issue, why would she accept that? Even if she feels the MC might be in the wrong, she should at least investigate the situation herself before she knuckles under.
I can understand her willingness to sacrifice her own happiness for the MC, but this goes way beyond that. She's not only (apparently) agreed to subject herself to Tybalt's whims, but she seems to have somehow missed that the MC would object to never seeing Jill again. Jill just comes across really badly here, which is a shame because the situation could be made to work if DPC were welling to settle for less injected drama.
