So... after all the hand-wringing about getting tossed into EHell, I have an update.
Last night, after MIL left from her visit, DH and I intended to send the note. We sat down, he double checked everything, folded it up with the check inside, addressed and stamped the envelope - basically the whole shebang.
We talked a little before taking it out to the mailbox because I wanted him to be absolutely sure it was what he wanted to do. He sighed and told me he was tired of people in his family telling him he "couldn't do this" or "couldn't do that" and the fact that his mother had such a bad reaction is part of why he was so committed to sending it. Because, seriously, not her fight. But he looked at the sealed, addressed, stamped envelope and basically said 'ya know, obviously I can do it if I want to - here it is - sealed up, signed, dated, ready to send - and no one stopped me from doing that'.
Then he took the letter over to our safe, locked it up and said there was no reason to send it. He already knows where things stand and he's walking away from it all. And if anyone brings up his father to him, he's first going to bean dip, then draw the boundary, and then walk away if they can't get their heads wrapped around his decision.
So... black hole it is. I fully expect some e-hell worthy stories at Christmas time when we don't go over to FIL's house on Christmas Eve, but for now things are finally calm. Thanks everybody for the advice and the listening ears.