BuddyPress.org

About 3)

quick and dirty, maybe could been added as feature when user registers and has made comments before

‘UPDATE wp_database.wp_comments SET user_id=’User_ID_our_member_got_after_succesfull_registration’ WHERE comment_author_email=”his/her_old@used_before_mail.com”;’

edit: Is it normal looking at bp_blogs_record_comment() function for a chance to import “properly”?
http://phpxref.ftwr.co.uk/buddypress/bp-blogs/bp-blogs-functions.php.source.html#l227