Other mistake: There's a typo in the end credits - an extra S in "3rd Asssistant." (02:16:30)
Other mistake: When Finn and Rey see Han talking to Ben on the bridge, they look directly down. But the platform they're standing on is to the right of the bridge. If they're looking directly down, they would be looking at either the floor or the chasm around the bridge. They should be looking to their left.
Chosen answer: The actors playing Rey, Finn, Poe, and Kylo Ren were mostly unknown, while Mark Hamill is famous worldwide as both a screen actor and voice actor. The billing order was likely based on their fame and then on their new roles. It's not a union thing, minimum wage in the Screen Actor's Guild is far lower than what they were paid. It's a combination of bigger star, better agent, and the future of the series.
Greg Dwyer