As for rape:
Deuteronomy 22:25-29
“But if a man finds a betrothed young woman in the countryside, and the man forces her and lies with her, then only the man who lay with her shall die. But you shall do nothing to the young woman; there is in the young woman no sin deserving of death, for just as when a man rises against his neighbor and kills him, even so is this matter. For he found her in the countryside, and the betrothed young woman cried out, but there was no one to save her.
If a man finds a young woman who is a virgin, who is not betrothed, and he seizes her and lies with her, and they are found out, then the man who lay with her shall give to the young woman’s father fifty shekels of silver, and she shall be his wife because he has humbled her; he shall not be permitted to divorce her all his days.
IMO, betrothed or not, she was still raped and in both instances, if there was "punishment" availible, then there should have been.
So Person A is "humbled" by Person B (although I don't think this word fits with being raped) and is not engaged, it doesn't mean that Person A wants to marry Person B (obviously) to avoid shaming her family but she has to.
If these laws were solid today and if I was raped, I just couldn't.
But, being engaged to be married and then raped, I can see why this was considered the "greater" case.
As for the slaying of Shechem and his men (which brings the phrase "Guilty be association" to a whole new level), I think they (the brothers) would have done so regardless if Dinah was betrothed or not.
Sometimes people feel as though they don't "deserve" (in this case being they are above it; ferior) whatever it is that they have because of who they are related to, this is partially why I think Jacob's sons reacted in such a way.