So. The report from the farrier.
As I suspected, Tristan's hoof was grown out to the point where its flare was putting additional pressures on the hoof wall, separating them even further. This would be exactly why I asked the farrier in MA to trim him before we left.
The farrier will work on Tris in the morning. The holes are expanded to the point that he fears there is a very real chance that as he defines to trim, a chunk will fall away entirely. If that happens, he will have to rebuild with epoxy or by casting the foot.
I have my fingers crossed that his naturally good hoof strength will help him here, and the wall will hold. I don't know whether that's a pipe dream or not. After a day spent on the edge of my seat waiting for a diagnosis, tomorrow might be even worse as I wait to hear whether Tris still has all if his foot or not...