From everything I have read about Chinga, it sounds like to me that it is a way he has learned to evade the bit. I would work him constantly using only 1 rein at a time. With his issues, it sounds like most of them stem from using both reins. I can't remember but I think you ruled out pain for all of his other problems, right? If it's not pain, then it is habit. I wouldn't concern myself with the canter until you got the bucking issues under control. Then, when you start cantering, start with medium sized circles. Keep his neck bent just a hair to the inside and if he starts to drop his head, pick up the inside rein and outside leg to urge him to keep his speed and level out. I won't guarantee that this will work but it is something to try.