Fort off he didn't go by Calvin Clein because he mom called him that because it was written on his underwear and he told everyone to call him Marty. Second, he didn't get any memories from the new history because when he gets home he has no idea why everything is different. So none of these theories work.

The original Doc dies in part 1, Marty travels to an alternate future where everything has changed. He can never go back to his original timeline because it ceased to exist. The original Doc had no letter because you see him get dressed and there is no vest. When Marty interfered with his parents he erased his original timeline and created a new one when he saved his parents relationship.