He's on heavy pain meds, and he's not allowed to jump up or down on anything for a week, so we locked him in the kitchen via a baby gate. The consensus is that somehow he injured his neck, but since he responds to pain medication and doesn't have any swelling, that he somehow injured himself when he had stitches. I theorize that he was on the couch or something, jumped down funny because of the staples on his stomach, and landed on his fat head or neck.
So... if he heals, he might have another go for a few more years. His stomach, guts, and scar are fine. The removed the staples, put in some final stitches, and said the wound looks clean and healthy. He eats, poops, and still wags his tail, albeit feebly, at people who come see him.
Thanks, humbly, for your prayers, everyone. They work.