An interesting hypothesis, in the capacity as a researcher in the field of psychology (with a specialiity in coping), I must admit that I am sceptical towards "syndroms" that tend to involve a great deal of reductionism, e.g. attributing all of Jones' personality traits to his place in the sibling group. I am sure you would agree. But I have also given a lot of thought to Jones' personality in relation to his siblings. What is so special in this case, is the tremendous success and athleticism of his brothers while Jon found himself lacking the same traits when playing football growing up. So whether or not it is due to the "middle sibling syndrome", I would agree that his most important "significant others" are his brothers, and he could hardly have come up with a better way to "out macho" his brothers than to become a UFC champion.
I'm also inclined to agree that he tries to hide his low self-esteem by over compensating. But again, that is why he is such an exciting and unpredictable fighter.
Btw - the Gus fight was the only fight in which his game plan to outdo his opponent at their own game, failed. He succeeded again with Glover, but I am sure he got a taste of his own limits in the Gus fight. If he had planned differently during camp and emphasized his own strenghs, the outcome may have been quite another...