No, you're not being irrational. Why did he text her to see if she was ok anyway? I doubt it was about the venue, it was probably that there is still something there, at least on her side.
He hasn't been totally transparent with her lately that we're talking again so I feel like there was an ulterior motive there and maybe she also left because she felt uncomfortable meeting me
If there is and he hasn't been upfront about you two working on your relationship, so he allows her to show up and you are there, what's that about?
Was it inappropriate for him to invite her, yes, when you two are just trying to work things out.
Have you considered that using drugs or not, maybe he just is not the best guy for YOU? If this is playing out the way it appears to be playing out, this is high school stuff, is he generally immature?
The whole story is weird to be honest. I'm not sure what he is getting out of this except maybe an ego boost.