I agree with your scenario. I think that the fact that the accident happened under RZ's care didn't sit well with some family members, and I agree that many of them didn't like her to begin with. I think JS and her were having problems prior to the accident and that this was the last straw, as you say.
I also think the Shacknai family knew from the start that MS wasn't going to make it, but they waited to inform RZ. They might have just found out that everything was finally together for the donor program at a late hour, and that meant they finally had a time frame within which MS would be taken off life support. That's what I think the late nigh call to RZ from JS was about, and that's when he told her that he wanted her gone before he got home the next night.
I believe she was furious because she thought she should get some kind of credit for "saving" MS until his family could say good-bye to him, and decided to commit suicide publicly and nakedly in a way to either humiliate JS and his family, or to somehow atone for MS's death. I think the shirt around her neck was for vanity's sake. Suicide experts say that hanging is the second way most women commit suicide (the first being pills) because they don't want their appearance marred.
I think her note was meant to be a jab at JS, as in "I saved MS until you could say good-bye to him, can anyone save me?", almost like she wanted credit to the end for "saving" MS even though she didn't save anyone, certainly not MS.
I also think she went out in the most jarring, blazing way she could think of in order to get back at the family (trying to ruin their family vacation home for them) and also trying to detract from MS's imminent death.
I have a relative who is a psychiatric nurse who works with the criminally insane, and she has her own theories about this, but I'm not sure if I can post them here.
Everything I've written here is only my opinion, but I believe it was suicide. However, I am open to the fact that there may be more information pending in this case that would change my mind.