I thought that Marvel established, as you say, that you can't change the past. But didn't Captain America do exactly that by altering his own timeline and showing back up as an old man at the end of the movie? The implication was that he went back and lived his life as if he had never been frozen. Is Marvel breaking their own rules here or am I missing something? It seems like he should not have shown back up in the same timeline if Marvel were following their own rules.