H.H.
she is probably thinking "If I keep harrassing the g/f she will give up and break it off then I can get him back". Until you are married this will continue happening. I have seen this in other relationships, because they decide after a while they really do want their husband back but now their husband has moved on and in their minds it's not too late until the new relationship is married so will try all they can to break up the relationship to try to get their life back. Until you are married there will always be hope for her to get him back. Once you get married she will realize it's over and may still give you problems especially if she sees that you are bothered by them but she will have to come to the reality that it's over and no chance of getting him back.
You are doing the right thing about being positive to their daughter about her mom. The worst thing about divorce is watching the kids feel torn between their parents and having one parent bash the other in front of them and then the child feels they are worthless because they are part of that parent and they also love both parents no matter what so is good that you try to stay positive for her sake.