Red-1
I used to be decisive, now I'm not so sure...
I also don't like things that I don't understand. I would be asking a good horse vet exactly what was going on with the cracks. If they were unable to supply a satisfactory explanation I would ask them to send very clear pics to Dr Knott so that I was doubly sure everything was fine. I am sure many may find that over the top but better to be safe than sorry IMHO.
Re the booting to get him out riding what would most likely work would be the largest scoots that you tried that twisted. I expect you had a fit kit. I never use them for scoots I don't find they are accurate enough. If you have the real thing with straps on the front it will probably twist a lot less. To prevent damage to the back of the pastern/heels I would use the endurance gaiters. (not the gaiters that come with the boots) The larger size boot would leave sufficient room for them and with that combination the heels should be safe.
Vet booked for next Thursday.
The scoots would have rubbed him raw, even the large ones. His heels get dandruff and are not robust!