My take on this entire thing has been more like what you've just posted.
I believe when Brian assaulted Gabby in Moab, and it was so public with police involvement, she decided it was time to end things. I believe Gabby sent Brian home while she decided what to do next. That's why he was gone for a week and was moving her things out of the storage locker. Somehow Brian managed to get back into her good graces, but I think by this time he was seething with anger.
He flew back, promised to keep himself in check and treat her better. Then the fight at the Merry Piglet made her realize he wasn't going to change, she told him it was over and he drove to a private location and killed her.
The most dangerous time in an abusive relationship is when the abused person attempts to leave.