It seems the only film that had the least debateable things was part one. Cause in both 1985 Marty sees Doc get shot before he leaves to 1955 so even if doc knows about the libyans, Marty still thinks he has to warn him in 1955.
The second one always had me wondering why biff returns to the original 2015 with the delorean if he changed it instead of going to his rich 2015, but a cut scene on the dvd shows the old man fading out of existence (maybe to wealthy Biff's 2015?)
I agree about the tombstone photo in the third. Who would Marty travel back to 1885 to save doc if he doesn't see the tombstone?