Another couple of ideas for the cooking pot although it sounds like you know what you are doing and have tried most things.
Take out bleed nipple and add some thread tape (plumbing supplies), stops any fluid or air passing along the threads.
Open bleed nipple a few turns and pump lever fast a few times making sure the res doesn't run out. This used to work on old Land Rovers as a small bit of air would trap inside and need forcing out.
Sounds like you've tried positioning the calliper so nipple is uppermost, also pushing the pistons back in and locking them in place whilst pumping lever. This works for poorly designed callipers where the nipple is not at the top/uppermost point.
The tube from the nipple should also go up for a bit before going into the jar/res so if any small bubbles do come out they will keep rising.
All else fails hit something with a hammer - old Land Rover stress relief trick...
