I don’t think it’s a pathetic question at all, and I also think you’re taking a very mature approach to the circumstances. MHO:
Making things less uncomfortable: primarily, time. I’ll also second the neutral small talk. If you haven’t been talking at all, however, jumping into small talk might also be uncomfortable, in which case (if you feel up to it) pulling him aside at some point and calmly explaining that while you didn’t part on good terms, you don’t want to involve your mutual friends and thus would appreciate finding a way where you can be comfortable with each other in public might be advisable. I’m a big fan of communication. If for whatever reason you can’t do that, and the situation comes up with mutual friends, explain that you’d feel better not discussing him with them so as to not strain your friendship. This will also help in the splitting of friends.
Friend-splitting, sadly, is sometimes unavoidable. I lost a few friends after one very bad breakup. You can take some small solace in knowing that people who would not listen to a reasonable discussion on the subject probably weren’t friends you wanted. But again, don’t trash him to the mutual friends, and you should be all right. Most of the time people will be cool about it. It’s also easier to advise in this sort of situation if one has specifics. I know that my tactics have worked by and large with my friends, but I doubt we have the same friends, so YMMV.
Also, as noted, I have been in a similar situation. The people who were primarily his friends (I had no real association with them outside of him) drifted out of my life. No real harm done, as I didn’t really know them all that well. There were a few I would have liked to have known better, but I’ve accepted that I won’t get that opportunity. The people who were primarily my friends weren’t all that impacted by the breakup (the insane things he did afterward were another story, but that doesn’t really relate to your situation). The truly mutual friends by and large came to me individually and said things along the lines of, “He told me his side of the story. Not all of it jived, so I wanted your side. Do you mind?” I would tell them my perspective, pointing out places that I was aware of where his interpretation was likely different. They generally thanked me and we didn’t talk much about it again. It just didn’t impact our daily interactions. And, until he went all psycho on me, we were pretty civil in group interactions once things settled.
So far as his new interest, well, the only thing I can advise for that is time, and involving yourself in other things while they’re at the same place you are. Talk to other people there, and if there’s someone particularly understanding, explain that you’d like to be distracted for a little while. I’ve always found that to help. If she’s understanding, let her know that while you are very happy for her (and do only say this if you’re sincere), it’s difficult for you to be around the two of them together right now. Suggest a time where the two of you can get together without him around.
I hope that helps. This approaches more long-winded than I tend to be, so I sense it’s time to wrap up. If there’s anything more I can do to assist, feel free to post more here or get in touch with me via ICQ/AIM/e-mail. I do hope things get better soon. That sort of situation is never fun.