Time Travel can affect the timeline. But only for the time travelers.
This means Marty's mom is still a drinker and Marty's dad is still a turbo nerd. And Biff is still a asshole. Jeniffer is well... Jeniffer. And in Part 3. Doc is still dead.
And in the game Doc is still dead. But he never meets Edna. Only in what Marty see's. So if i were to hear Doc is dead. And Marty was to prevent that. Doc would still be dead in my world. Where as in Marty's world he is still alive.
Isn't that freaky?