It kind of sounds like you're not yet ready to be dating
other people...at least not seriously, and now that I think about it you never said New Fella was a serious thing, so that may be a bad way to start.
I do think you recongnize that it is time to move on and that will be impossible if you let yourself get so worked up...even to the point where you ask your ex to avoid you.
If your ex approaches you and New Fella, then say hello, introduce them and tell him that you hope he enjoys the show.
If New Fella approaches your ex and his friends...same thing. And as far as PDA goes, do whatever is appropriate for the venue and normal for you and New Fella and don't worry about your ex.
In other words, act normally and everything will be fine. And yes, you are overthinking this. Once you get through this evening with your ex and New Fella, it will be easier on you if/when you run into him in the future. It's an important healing step, so that's the good news