okay nailed the append (for now) that was easier than I thought. Gonna have to revisit the size calculation (probably as part of a greater refactoring of the MailHeaderImpl which has become a mudball). Regardless sending and receiving with IMAP through T-Bird (at least to myself) is working. Lots more to do (like delete command for instance), but this is progress!
As far as I can tell every major operation in Thunderbird works except for search. You can now move things, delete things, send mail, receive mail, etc.
I still need:
Following that we'll have reached the limits of what thunderbird can do with imap.