It’s simply because ActivityPub lives on activitypub.rocks, and the decision was made to maintain the forum, from the start, on this domain. The SocialHub was born from the APConf, there was no reason to put up another domain.
That’s because nobody (except my single commit there) wanted to tackle a website in Guile. We do have access to the website sources.
Yes, certainly can do that. We would need to subscribe an address and add a dedicated category. This would facilitate discussion here and there.