Some planning progress, finally: farrier will be out early next week, probably Monday or Tuesday. I just chatted to the vet, and we'll all meet at the barn to do a full workup.
Schedule will go something like this: pull shoes, do x-rays, have vet and farrier look at the x-rays together, tranq him to trim, possibly do another set of x-rays (I'm budgeting for it if they want, to check angles etc., if they're all going to be there might as well do it 100%), then keep him tranq'd to put shoes back on.
My worst case right now is a mechanical founder with accompanying soft tissue tears. I think he'd be more lame than that, but the shape of the foot is so awful I am afraid of coffin bone movement, which almost certainly guarantees soft tissue strain or tear. If it's severe enough, he goes into crisis management mode and we use a recommended local farrier for regular trims every 2 weeks or so to micro-adjust and try to compensate.
My best case scenario is simply major discomfort from underrun and long heels, combined with the flair and the missing hoof wall from the abscess, and the farrier is able to take significant foot off and relieve a great deal of that. My magical scenario is an instant cure but I think he's been too sore for too long to expect that.
What will probably happen is some middle ground with waiting: x-rays show clean but there's clearly soft tissue strain, and the foot can't be entirely corrected with this shoeing, so he may be on a longer-term painkiller to keep him moving more easily until the next shoeing. I'm about 95% certain that the worst of the missing foot will be gone after the next trim, in another 6 weeks. I think it will be April or May before the top hole - at the coronet band originally - grows out. I think it will be another 6-9 months after that before the scar tissue from the coronet band stops growing out lumpy. (It's easing up a bit already, thankfully.)
Someday I should write this all up clinically and submit it to a vet somewhere, because wtf, people. Worst. Abscess. Ever.