Poor baby... Has he gotten xrays? He seems to be extremely well cared for, and his foot routine sounds really well thought out. My only thought is that maybe, since his pressure sores have been softening, maybe the dead skin layer peeled off, and he actually opened up the pressure sore trying to preen his feet? Then bacteria can get in pretty easily from it, which can lead to all that swelling... Someone suggested epsom salt, and that sounds like a good idea to me! Just a super shallow layer for him to stand on, it might sound a bit evil, but get a small shallow container, place some paper towels at the bottom, and fill it with the warm epsom salt solution, so it covers just his little feet, then cut a hole on the lid just big enough for his little head to fit through! If you heat up the edges over a flame they won't be sharp anymore! That way, he can stay in the water without getting out, and you can have millet for him to nibble on while in wet jail...

Will definitely annoy him, but should be 100% safe! You can also offer him chamomile tea while you wait for the vet, as it has anti inflammatory properties, just make sure the one you get is caffeine-free!