Flying Spaghetti Monster wrote:
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.
Like he says, he figured "what the hell?!" once he knew the information he could then decide what to do with it. But he obviously must have read the letter in 1955 because it'd be hard to reassemble and tape together the letter without reading it. (But, I guess, not impossible.) It may have been the next day when things calmed down he figured there'd be little harm in it.
It would have been nice if in 3 we had gotten a nod to the idea that Marty's plan will still work. With Doc picking the coat up off the ground (wearing it that morning in the cool California in November desert?), finding the scraps and getting a look of "You know..." in his head.
It's also "possible" knowing the danger he was in Doc always WAS wearing the bullet-proof vest, Marty just never got the chance to see Doc survive the first time around because of the Libyans. (Doc remains on the ground, unconscious, in both versions of the time line through the DeLorean/Bus chase and time travel sequence. And in both cases Marty gives Doc a cursory check, the second time around "our" Marty apparently not noticing the lack of blood on the corpse or the bulky BP vest on Doc. So he must have "felt the same" as he did earlier.)