I can definitely sympathize. I am going through something similar with a 13 year old horse with multiple issues. One of the reasons I have not done it yet is because I know I will get flack from the vets and other boarders. Vets here are extremely reluctant to put anything down that is still up and eating. Horses are extremely stoic creatures by nature; I am convinced they are sometimes in much more pain than we realize. My first horse I waited too long to euthanize too (due to a vet’s convincing me it wasn’t urgent) and like Halt, that guilt stays with me.
I think the vet is unrealistic and out of line in her offer. What is the purpose in putting this horse into training…to see how much he can take before he breaks? I would be leery of giving him to her in any situation because it does not sound like she has a grasp of the gravity of the situation.
I wish you the best of luck and peace in your decision.