So much of your story and current sitch seems so awfully similar to mine, with genders switched. My D was final in April too. And I am struggling to keep from going insane as the OM keeps worming his way into my S's lives. I enjoy my time with my boys very much, but there came a point during a swim in the pool with them last weekend, and they were going on and on about how "Mr. OM" did these special fun activities with them in their pool, that I stopped what I was doing long enough to ask them both to just cease comparing what we do to what he and xW have been doing. I have not really been so sensitive about these things in the last few months, but in the last week this was getting to be too much. And I know they're just children and don't fully comprehend the impact of what they're saying, but unless I said something to them how else are they to learn to not be so insensitive.
I am now deeply resentful of xW for allowing her f'ing BF to roughhouse and get physical with my S's, especially in a shallow apartment swimming pool. She is again running counter to the safety instructions I have been instructing our S's (no jumping or diving, for example). And I find it highly inappropriate -- and not just because I am jealous for my S's.
So I really understand your mixed emotions -- you want your children to enjoy a lot of positive experiences, but not with people who have been proven to be morally bankrupt.