Some good ideas here antiorario.

1> You'd have to ask Ryan or Lyle as I didn't write the uc_cart module or post an bug issue.
2> I like this idea. It will be easy to implement as well. I'll try to fit it in to uc_roles before the next release.
3> I guess I'm not seeing the usefulness of this idea. Users can already hold multiple roles. If one role expires there other roles that the user could already have. Is there particular instance of where you might need a set of mutually exclusive roles?
4> This is already possible. Admins who can administer users can go to user/*/edit and edit the expiration date under the "Role expirations" fieldset.