BuddyPress.org

At that point, they are just members of blogs, so yes, they have access to the dashboard with options according to their blog role. Asfaik, P2 is not meant to be the only post option as it meant for quick posting and only text. So it would greatly limit things by having that as the only post option.

For your situation I would not use the groupblog plugin, but stay with either wire or discussion forum. Also, to answer your other question, bp-groupblog does not work wihtout buddypress.