So you’re saying that in that ending scene, right up to the point where Doc gets shot with bullets and falls to the ground, he’s really dead and didn’t have the letter or the bulletproof vest on or any of the knowledge about Marty having been in the past…
…but as soon as Marty and the car disappear (going back into the past), all that stuff suddenly appears magically in 1985 (the letter, the vest, and the memories)?
Maybe. I see what you’re saying. But it would stand to reason more (I feel) that Doc just already knew what was going to happen (at that point), hence one of the major reasons for showing him and explaining all that stuff to him (including telling him the precise time to be there and even waking him up and telling him to hurry when Marty fell asleep earlier in the movie–at the beginning)
(although one could argue the first time it happened, it was definitely all real. Me, I’m one of those who believes more of the “Whatever happened, always happened” type…in which case, if we had seen a few minutes after Marty first leaves in the car (at the beginning), we’d have then seen the 2nd, “later” Marty come running up—we just didn’t see that part because the film cuts away and follows the first Marty’s POV)