1) Nothing wrong in principle with 2 char usernames, we just chose to require 3 chars minimum for user-self-created accounts.
2) The preview page is (currently) central to a number of operations - though this may change in the future. Is it the appearance of the disclaimer text itself, or the two-step process for posting a message that's annoying you most?
3) Yup, the UseNet/Email .sig standard is '-- '. However, this is not UseNet or Email :-)