Your question is a tough one, but I think I can explain some of the inconsistency. For starters, I think I see two types of inconsistencies:
1. How he acts towards you alone (friendly/aloof/flirtatious/sulky)
2. How he acts towards his friends (friendly/posturing/lying about you)
I think that #1 is always more true than #2. e.g. I think that in the example of him seeming to have fun at supper and then telling his friends that he didn't, that he really did have fun, but was trying to impress them somehow. All the examples you gave of how he interacts with others suggest (to me) that he isn't genuine with them at all, and wants to use them as a distraction - that is, he uses them for a good time (partying, laughing, etc). You, on the other hand, are someone he can be a little more deep with. In fact, I think he was probably being serious when he told you that it wasn't easy for him (and I came to that decision based on other examples you've given). Admitting something like that is admitting weakness, and I can't think of a single ESTJ who would ever want to fake weakness. Why not fake strength?
On that note, my theory about #1 (which explains most, but not all, of the things you described) is that he's trying different methods of coping with your breakup and new status as "just friends". Each method has been chosen to minimize the amount of pain he'll have to go through. Leaving without saying goodbye? Chosen because saying goodbye is difficult and emotional. Being distant with you? Chosen because it makes things less awkward. Crazy partying with younger people? Probably also to numb himself. Of course, what it doesn't explain is the semi-flirting. Maybe he still has some feelings for you? I really don't know. I would suggest that you talk to him about his mixed messages towards you in that regard, but if I remember correctly, you already did, and he was totally uncommunicative. So I guess I don't have any advice for you there.
Now for the attempted answer to your question: I would definitely go. Who says you have to spend a lot of time with him? It would be fun, and if your ex gets to be difficult to be around, you can be around other friends, right? Is it possible to bring family AND go to the conference, or is it too short notice? Either way, if you talk with him about partying (or, rather, how maybe he shouldn't do it since it's a business trip), the friend issue would be taken care of. Can't promise anything about possible flirting or running hot and cold (because like you said, I'm not a fortune teller), but, speaking as a cold-hearted ESTJ, I guess I'd choose the conference, and if anything awkward came up, I'd find a way to deal with it and/or avoid it.
Pardon the late-ish reply. It took me a while to think about this/write about it. Definitely the trickiest question I've had on this thread. I hope I've helped a little bit, because I'm not so confident in my answer.