Maybe just enjoy one of the most enjoyable movies of all times, and stop beating it like a dead horse.

If you break it down tons of loop holes, point is no matter what you do in past or future - you can't change events - you have to have character changes, to shape the events of your life...chaos follows chaos and misery loves company. Marty just grew as a person by living through time travel trauma - he out grew his old self and was not as immature. Yes Doc could of changed the event of the drag race by grand theft auto'ing the royce to prevent the accident all together and would of changed that one event, but there would be another and another and another. So he had to change Marty and yes, if you want to theorize doc being doc must of pin pointed that one event for Marty to have to recognize and go through including his letter eventually on that same time line stating I found a life in 1885 deuces, which would trigger Marty to save his life, If you when Doc returned from the future to pick up Jen and Marty he was a changed man, like he had spent a good deal of time , time traveling and living in the future...he was well adjusted. So Docs ambition was not a plan made off on a whim, plus you see just how fragile a time rift is as it's altered as the have chaos every time they enter a new time era..anyways let the dying horse rest cause you could beat the heck out of it.