BuddyPress.org

“1. Do these blogs share the identities of users, or do you have to register for each one separately?”

The userbase is shared. This is the part BP inherits from MU.

2. Disable how? Either BP is set to run sitewide (and show admin bar everywhere with the profiles in one spot) or it;s set on one blog, thus not appearing on other blogs.

3. short of writing a book, I’m not sure what else can be added.