Which flags particularly, americandreams?
With the new info on your ex - and the recent interaction I have had with MINE these last few days - I would also agree that perhaps, as he is thinking there is more to this party than you are prepared to accept, that it's not a good idea to attend.
I still don't like the DP threats of ending it unless you agree not to go, and still think you need to do some very hard thinking on the way he is trying to control you for his own insecurity. Trouble is now, you are damned if you do and damned if you don't!
If you don't go, P thinks he has won and it potentially sets you up to more little trantrums when he feels like it, on the other, you defy him, go to the party and into the clutches of the Ex, he makes an arse of himself, you suffer and then DP says told you so Argghhh! 
I think the best thing, if you get on with them, is to call your exMIL and talk to her about it, say you wanted to come, but that EX is under the impression that there is a reconciliation on the cards, and there isn't.
Make it clear to all (including your P) for that reason AND THAT REASON ONLY, you may not be able to attend.
Tell P that you have made a decision based on the situation and not on his threats to end the relationship, and if it were not for the ExH saying you were an item, you would go whether he was happy to 'let you go' or not.
Make sure that you take an equal partner stance in everything you do from now on. Remember that you don't need to ask his permission to do anything, and you are more than capable of making your own decisions. You will hear his opinion, but the last word in your life is YOURS.
You were brave to get out of a very bad relationship. Jury's still out on whether you are in a much better one, but in any case, you owe it to yourself and to your DS to be in as strong a position as possible.