Okay, partially right on all counts. *heh* Mark had his accident in January of '77 on his way to shoot pickups for Star Wars. George had early notes and his first treatment for ESB (mid-to-late-'77) have Luke crash or get whacked in the face to explain scars that nay or may not be there by the time filming commenced. Regarding the accident (in Rinzler's Making of ESB):
He wallowed in self-pity for a bit, but eventually came back around. As he healed the differences became minor, and he and George felt they were easily explained by post-adolescent refinement plus several years fighting for the Rebellion and probably occasionally being injured. In other words, Luke grew up between films, and I think this actually helped George take that tack. Still, considering principle photography didn't start for another two years (and change), he had plenty of time to heal.
--Jonah