OP, I have not read this entire thing but am going to as I own this exact horse. I mean the whole package. Except he’s a 17 hand, 11 year old Holsteiner. I love him like my child and he runs to great me. He too is opportunistic with an open stall door and although he doesn’t break away, he can be a kite to bring in but not because he’s an ass, he’s just so dang happy!
The long spot is the death of us. If he gets funky to a jump or a long spot, you will pay for it on the backside. Like your guy, his ears are not pinned. He’s not pissed.
I too gave up last year and we just flatted.Then a girl came to my barn and her mare bucked so much worse on the backside and she was still riding so I thought, screw it, we are going to get started again. We started at square 1 with an 18" cross rail CT (we are eventers) in March of this year . I found that half halting him to the base so we had no long spots greatly reduced misbehavior on the backside. We finished at Novice and he got reserve champion of the year. We also got champian at Novice for the CT series (novice is 2’11").
I managed to get hooked up with a great clinician, John Michael Durr, and he is taking him next week for 3+ months to Aiken for training. During a cross country clinic with him last month, we jump a type of jump that we had never jumped, one that had a ditch under the jump. He landed on the backside and went into a bucking fit and bucked me off. I got my self together, got back on and finished the clinic but he had stepped on my arm and while it hurt I wasn’t injured. His comment was “I hope he tries that with me!”.
With your horse and mine, it is a dick move not based in pain. I too have had him worked up for everything. Best of luck . It’s a good thing we love them!