I’m not a sys op so most of it is over my head, and because I have not been interested in a multisite I have not read most of it, but there is this howto which may help
Currently multisite forums are separate, That is, they may have the same “owner”, but they are not cross-linked. So any “hub” would require a lot of manual work
It would depend. If the forum was a fork it would be a lot of work. If done with plugins less so.
That may be the simplest approach. Categories specific abilities have been requested by many and apparently is on the todo list
https://meta.discourse.org/t/category-specific-moderators/11668/63?u=mittineague
But that would still leave the problem of having a massive number of categories