Re: set default Role of new user
i had the very same problem at http://buzzdev.org that default role on main blog was ignored for new user signups.
I am running plain WPMU 2.7
Looked into the source code and solved the problem by editing wpmu-functions.php at line 672 (function “add_user_to_blog”):
Now whatever default_role i set in wpmu/options.php it gets applied to new user registrations.
http://buzzdev.org (wpmu test)