...he's one of those guys who loses interest in girls as soon as they gain interest in him...

This isn't one of your questions, but this is a huge red flag to me. If someone loses interest in me because I'm interested in them... just, what the fuck? That doesn't make any sense. You shouldn't want to be with someone like this in my opinion.

1) what is most likely going through his head, if anything, about the relationship right now  (3 weeks after breaking up )? Is he completely over it?

Probably not totally over it. You know him better than we do, though, and you have the same extroverted function we do (Extraverted Intuition). What's your intuition telling you?

3) even if we don't get back together, I'd like to be friends with this person in the future and share with him all of this mbti stuff. Considering how I freaked out on him, would an intp maintain a friendship with someone they saw act that emotionally and irrationally?

I would, but I would also be worried about putting them back into that emotional state. He might feel like he can't be completely honest with you and has to walk on eggshells.

He also said he loses interest in girls who are interested in him, and I think you've made it obvious you're really interested in him. He enjoys the thrill of the chase more than the actual relationship, and it sounds like he's already got you... I want to reiterate my earlier feeling that this isn't a relationship worth pursuing. It sounds like a futile effort.

Good luck. You seem to be a hopeless romantic, and I promise there are other romantics out there who might be a better fit for you than this emotionally unavailable guy.