From the writers' official FAQ
, mandatory reading for every fan:
Yes, thank you. I have read that before.
As I stated, I am unable to rectify that glaring error in the film. As Zemekis says, they goofed at the end of BTTF Part I, mostly because they never intended to make a sequel. That scene at the end pretty much pigeon-holed them for Part II. The plot about the "kids" was pretty thin, which is why they had to beef it up with the real plot, the almanac.
Internally, following the logic of the BTTF universe, I am OK with things like the photographs or newspapers disappearing/changing. Those things are relics of a future time (that now exist in a past time), a time that is in jeopardy based on events that occur in the past. But in BTTF Part II, they are physically in that future time, so nothing they do there has any effect on, well, really anything.
It was cool to see the future (the whole point of making the movie). But if this were reality (hahaha) a better thing for Doc to do would be come back at the end of Part I and say, "Marty, your kids turn out to be assholes. In 30 years, make sure you keep an eye on them." Takes less jiggawatts that way.