A huge red flag on this horse is that it seems to have deteriorated rapidly once it was in the US. That seemingly steep rate of decline could continue. Although we can’t know exactly what condition he was in while in Europe.
This is the thing about the vet and/or the trainer, exploring options for you to move forward with this horse, in spite of KS.
Neither seems to have experienced the reality of a KS long-term, in their barn, under their saddle. If you have a serious abnormality in the bones of your foot, can you expect to run a race on it – or even conduct daily life in the same normal way as people with two sound feet? No and no.
A horse with KS may not always be symptomatic. But when they are, the rider may find out when they mount and carefully ease down in the saddle. And the horse explodes in a buck that sends them airborne at height. To re-coin some popular phrases, the rider may experience an unscheduled rapid descent from a height of 16hh or 15hh, followed by the abrupt disassembly of various internal body parts on contact with the ground.
Just for what it is worth, it is not uncommon for experts, from vet medicine to the sellers of household fixtures such as dishwashers, HVAC, etc., not to know what it is like to live with it over the long-term. They know what the literature says, what one-time studies say. But their longer-term experience is limited to what they own themselves in their personal lives. And it may not be this.
When there is a lot of talk about KS rehab, targeted exercises, wet saddle blankets, etc. & so on, I always wonder if this is just technical talk from reading papers and websites. By someone who, no matter their expertise on the surgical end, has never managed a KS horse in their own barn over months and years. A horse that is sometimes sound and sometimes unexpectedly not. Someone who has never seen a KS horse suddenly violently pitch a rider in a highly dangerous situation. Sometimes seemingly out of the blue, as the horse may have been going ok for past rides, or even on this ride, until that moment.