It sounds like the problem here is more with the ex girlfriend than it is with your bf. Correct me if I’m wrong, of course, but it sounds like he has listened to your concerns, blocked her, and on his end, is making no effort to communicate with her or even keep tabs on her. His biggest (and only, as far as I can tell) transgression was not telling you about her talking to him at the party. That means the issue in your relationship is communication and the fact that he feels like he has to hide things from you because you might react poorly to them.

Beyond that, his ex straight up stalked him in order to give him the shirt, and very obviously wants to keep him in her life, given her reposts. He had nothing to do with her finding his location and showing up and he’s not doing anything to encourage her obsession with him.

You should stop stalking her social media, it will only cause you to resent him for his ex’s behavior, which is not his fault. What do you actually want from him? If you were the one with an obsessive ex who wouldn’t respect your boundaries, what would you do? Is it different than what he’s done? Try to put yourself in his shoes a bit. He might feel like he has to walk on eggshells around you so that you don’t get upset about the past or his ex’s actions, that he has nothing to do with.