What did the Marty from the enhanced timeline at the end of the first movie do when he went back in time?

That Marty would have heard the stories about his Dad standing up to Biff, with or maybe without some passing mention of a Calvin Klein guy. He would NOT have heard the story about Lorraine's dad hitting George with the car. So he would not have put the pieces together with Doc, realizing he has changed the future of his parents getting together.

Lorraine still tries moves on Marty, Marty is still freaked out and leaves her home, and after that he stays away from her and also avoids the high school.

Probably Doc and Marty focus on the car situation. Again Marty doesn't realize he has prevented his parents from getting together.

Unfortunately Biff probably succeeds in SA Lorraine at the dance. George is dateless.

As Doc is finalizing the "weather experiment," Marty disappears, erased