To be honest, I think he's right about you being indecisive: you saying you don't want a serious relationship but then getting upset when he's been seeing other people. But then if he acted int he same way, you're probably not the only one who has been indecisive. So now you need to make some decision and stick to them. He obviously means something to you otherwise you wouldn't be going through all this so long after you split up.
So the bottom line is do you want a serious relationship with this man or not? After that, you can address the other things (although to be honest, I don't think you can really be that upset about the fact he dated someone else so soon after if you told him you weren't interested in a serious relationship anyway - why does he have to justify his actions once you're off the scene?). Is that why he's made you feel worse?
Once you've decided that for yourself, tell him straight up. Otherwise you'll continue to have these mixed communications which have, it seems, muddied the water already.
If you're not interested in him and all this meeting is going to be about is getting 'answers' about why he behaved in the way he did, then I think it will hurt both of you. I also don't think, as I mentioned before, that his actions POST your relationship, are really any of your business. Maybe he just met someone he really thought he'd clicked with even though he wasn't "looking" for a relationship? Or maybe he just wanted some regular s*x? It's really not your business. Leave him alone, stop texting him, delete his number and move on. For both your sakes - you'll only get hurt.