As for "fake fatman Jabba" being in costume, I think that was at Harrison Fords request, he liked to voice his annoyance & frustrations on set at the script & not having anything to act too so perhaps this scene would have been slated to be shot late on into the shoot & by this point GL would be doing anything for an "easier" life.
Interestingly HF took it upon himself to rewrite/adlib his lines in SW & ESB if he didn't like them!