Back to the future: So Marty returns to the present and his family is crazy rich and successful in the end all because of the events leading up to the enchantment under the sea dance. That part u get, but isn't that kind of profoundly tragic? I mean, Marty's family is so much different than what they were before Doc got them into that mess, that Marty, hardly would know them.
Also, the Libyans that crash their van in the twin pines parking lot probably would have survived that "wreck" they were in and then just shot Doc in the face.
Finally, even if those Libyans did die from the world's most tame fatal car crash, there would definitely be more. After all, this terror group got its hands on plutonium and operated in the United States suburbia. Certainly other cells would have gotten to them or found a way to get the plutonium for whatever doomsday device they were planing.