He doesnt back down from a fight.

Even with his first confrontation with Biff from the first movie to be very confrontational despite the unanticipated size of biff & I believe it was this that inspired him to influence his father. Despite being an absolutely hopeless cause of trying to fix the man who his father should have been only to be stepping into the shoes of where he wanted his father to be, to win over his mother who was unknowing of the fact until she had a sickening feeling from kissing Marty by stating it was like kissing her brother. Which he responded to explain that he new what that was like to feel the same way.

However through out the course of the movie he is confronted by consequence of altering time lines to change who he is which he hadnt experienced in his former timeline before the accident which lost the use of his capability to play the car which he valued, so iconically. Which shows that despite physical incapacity, that he would still confront his issues.

Marty didnt gain any memory from traveling back to the present of his newly found alternate time line. Having no idea that it was biff who was polishing a car at his parents house or that his father would be standing tall with swagger to be dominating his radient mum who he found unusual to see the way she was or any idea that his fathers new found wealth would come from writting a book based on the inspiration of Martys direct influence or any idea what the keys where for, for the newly promoted black 4WD that he wanted to not expect to see it in his garage. To have achieved success even though he didnt anticipated financial gain over his ordeal from helping his friend with something he cant tell anyone about because of how it could impact his life in addition to being warned by the doc about repecutions.

What did happen is that during the present time of being in the past. He had altered the time line to lead to him becoming none existant, however it is something that triggered in his father, that caused the change to revert Martys present state of being about to cease to exist which was out of Martys hands.

You simply dont know who he is, as the punk rock raskle who wont back down from a fight, to have encountered a life or death experience to want to reright the course of history as the time traveller that no one can know about. Not even his partner who he was shocked to see while having to adjust to his new life without her knowing that he is not aware of the changes as the Marty she knows.

So essentually you dont know what youre talking about and time enveloped around him, the doc and Einstein (who clearly dont know and you really have to watch the docs comments about).

Plus you go on to say about his triggered reaction with no comprehension behind the change of Back to the Future II, where because of what happened from him suffering from a horrible accident, that instead of reacting to being called "chicken", he decided to avoid confrontation and this leads

Marty has no memory of the change of his parents as his father is now a new man with confidence to stand tall and his mother is now a radient reformed version of herself and has absolutely no clue that his fathers success came from not only the book that he wrote to have acrude wealth but the confidence that he has from beating his bully and marty had no idea it was biff who was polishing the car at his parents house or even that the newly promoted black 4WD was his when handed the keys to have no idea what they where for, before discovering it in his parents garage to have achieved success despite not anticipating that he would have had financial gain from time travel which was rather more of an ordeal of a life time experience which he couldnt share with anyone, to be startled to be confronted by his girlfriend who had no idea about his former life, in order to persued as the Marty that she new.

Your problem is that you think that while everything while everything was crumbling around him, he became the altered state but its because he had changed the current time line, that the effect was about to have repercussions on his entire existance. However it is because of the steps he had taken with his father that his father changed the future with something triggered beyond Martys control.

You think its just because he had a newly found form of confidence to be out of charecter but you dont understand Marty at all.

Marty is the punk rock ruskle who doesnt take crap from any body but his life or death experience may very well be the newly formed confidence youre looking for, I think youre overlooking the fact that due to him being a time traveller to reach beyond what anyone can know about because of its potentual catastrophic events if someone where to take advantage of it, like the Doc had warned, its easy to say that he could take advantage of knowledge of the manipulation in time and the choice of song in this particular case could not be as impactful as u think.