you have a lot going on there, but it all boils down to the same old thing; the horse is spoiled and may not be very well trained to begin with. If he has been through a lot of humans, he has probably learned a variety of ways of getting what he wants. And the compulsive playing with a toy can be an idicator of a deeply felt anxiety. Is he alone? Horses alone can be very unhappy. Can he at lease SEE another horse nearby?
The thing that has to be done, and the trainer may be able to help you, depending on what kind of trainer it is, is to make it so that YOU are the most important thing to him. More than the toy. More meaningful might be a better word. You work with him on the ground first, either in a round pen or small paddock and/or on a long leadline. There are a whole series of excersizes that work with a horse to get him paying attention to you and giving you the respect thatq something meaningful in his life will get.
Right now, food is meaningful. The toy is meaningful. The kid on his back? no.
Even you are not meaningful to him. You are an annoyance that is intereupting his focus elsewhere. So, you have to get him in a place where you basically force a change in his thinking, which puts you into a position where you ARE more meaningful than the toy. He may still prefer the toy to you, food to you, but when you ask or even demand, he will know that you have the authority to insist and back it up. Once the horse believes this on the ground, they will be much less likely to challenge you in the saddle.
You will also want to make sure , via the most expereienced person you can access, that the tack, especially the saddle, is not fitting such gthat when he starts to canter, it hurts. That would definitely cause him to want to try and "run out from under his saddle".