I found myself wondering this time around how Marty and the Doc first met and fostered their close friendship.

I didn't compare Marty's room too closely from the beginning of the film and the end, but maybe I should try to get screencaps tonight when I go home--it basically looks like aside from the truck, Marty hasn't changed at all in the new timestream, which doesn't seem possible given how extraordinarily different everyone around him is.

though if you take later movies as canon

Why wouldn't you?