The answer to this other question is easier: grant admin rights on the forum instance. The grantee is then theoretically able to grant further permissions on that forum instance to others by herself, by using the /permissions/one page. This is only possible for already existing user groups and single users though, the grantee won't be able to manage group memberships.
The only problems with this is that there is currently no link to that page from the forums admin page (should be very easy to add), and that the /permission/ UI sucks, especially if you have many users and groups. There's interest to improve that but I don't know the current status of any efforts.
(btw., I am talking about the new forums package, not bboard, but that propably applies to both)
While creating one sub-site per forum may seem wrong, the concept behind it is that you have different sub-admins that are in charge of different sub-sections of your site, of which a part would be a forum instance. And managing group membership information is obviously much more than just managing a forum, so it makes sense that it is coupled with other rights as well (subsite administration).
