Just read through your thread and had a random idea.
When I really want something from someone I try and view it from their angle and see what they want. Your dad seems to be pretty into harness horses and seems to like spending money on them... could you find a trail horse that is also broke to harness? Something that would appeal to him and still be of use to you? If you found a horse that he wanted it would take much less convincing to get it.
Just an idea. I hope you get lucky and it all works out