Having got the Blu-Ray of the trilogy for X-mas, one of the things I noticed about part3 was rather odd.
In part one, at some point over 30 years (but likely close to 1985) Do decides "what the hell" and reads the letter. However, on November 12, 1955, he's dead-set against it. so is it that the next day, he's not steaming mad - maybe at himself - when he learns of his own fate by reading his own letter.
This makes me think that the theory of time travel in these films is comprised of separate layers. There is one reality in which no time travel ever happened. Then, layered through it somehow, is another layer wherein Mary travels to the past and meets Doc, and Doc refuses to read the letter until later, and that future resolves itself. The next layer assumes the other layer itself before Doc reads his own letter from 1885. My point is that Doc's decision to say "what the he''?" is not dependent on having read his own letter from 1885, and he would have come to that conclusion anyway, and the continuum would have assumed that that decision played itself out before mapping how Marty's return to see Doc the very next day might affect it. This works much like the logic of the second film, where in Mary and Jennifer can travel into the future but the time continuum assumes that they made it back to the present to grow up together and have kids.
Oh gawd, my brain hurts. I love these movies.