I can't believe I'm going to take her side on this but Rosamund and lucio were in a relationship first. She did not come in between anyone's marriage. It was due to the nobles requiring a noblewoman from a higher house to get the position of queen that he married another person in the first place. I completely agree with all the other things you've said. She was abused in her terrible household but she shows a lot of their personality traits as well. She is very greedy and won't be satisfied no matter what she has. She's also very cruel which is why she has no issues with destroying houses or getting innocent people killed.
I don't like Rosemud!! Yes I get it the pore thing was violated but that doesn't mean she has the authority to come into somebody else's marriage. Two wrongs do not make a right. And if you're going to tell me the Rosemud wants to be Queen, wants to start a War, doesn't want to be looked down upon, wants a title, wants to sacrificing an entire family just so that she can be satisfied with her situation then I would recommend therapy!!!!