I know it sucks to see another man in the picture. But our mind is usually our worst enemy in these situations it's like that song Mr. Brightside by The Killers...it's just setting off haunting scripted images of what they're probably doing etc.
Unfortunately it's also a reality check about our own progress or lack of it. We'll always be the slave to these bombs unless we let them go completely and set ourselves free. I wish I knew how to do this myself but I know with time it does get better.
I think you're right in thinking about callling your lawyer and moving on ahead with the process. The legal ending may help with moving on and you're 'waiting' to do things because you're not divorced yet including the 31yo.
I agree with the 31yo that you're a great dad and your focus should continue to be the kids.
Me: 35|WAW: 38|D: 6yo | http://tinyurl.com/2dxx7m6 Feb 2006, left, came back in two weeks Aug 2006, left again Apr 2007, filed for divorce Dec 2007, reunited Mar 2010, moved out, filed again