It seems pretty clear to me that they broke up by August 17, if not sooner.
Five days after the altercation, Brian flew home to Florida to move her (or joint) possessions from a storage unit, leaving her in a Salt Lake City hotel, returning six days later.
In the altercation, she was the aggressor - or at least the one who struck and left scratches. The reports suggest that he withdrew from the argument because it was getting too heated - first backing off and then getting in the van. It sounds like something like this had happened before, and this was a learned (or counseling-educated) strategy to de-escalate the situation. She had serious anxiety issues and, apparently, abandonment issues, so his withdrawal might have terrified her; after all, that was a stated cause of the altercation. Perhaps he did overpower her or abuse her at some point, but there's no evidence of that other than what we know about how these things often go.
Five days later, he flew to Florida and then back again six days later. In the interim, her father reportedly ordered Uber Eats "for the couple" in the hotel where she was staying; did Dad not know that Brian had flown back home? Did she not want to admit that they were broken up? (Or is my reference mistaken?)
All of this looks like a negotiated breakup - to this point, anyway. He flew back either to help her drive home (or drive on, if they made up), or possibly to drive the van home alone while she flew home - or somewhere else - alone: the hotel where she was staying served the airport.
I don't think things look good for her, but there is a possibility that she gave him the van to drive home and went somewhere else herself.
In a completely different scenario, probably mutually exclusive to the one above, I'm really intrigued that their altercation took place at the Moonflower Food Co-op, which is where Kylen Schulte (one of the murdered newlyweds) worked. Both couples were living in their vehicles, possibly in the same place if they encountered each other at the Moonflower. Perhaps Gabby and Brian encountered the newlyweds while dispersed camping, and were told that Moonflower was THE place to get the beautiful food in their video.
At a minimum, they would KNOW about the same places for distributed camping. That might implicate either Gabby or Brian, but if the two mysteries are connected, I lean towards a third party with some kind of a mutual connection, either from the Moonflower or from camping.
There are definitely some things LE is keeping to themselves, doubtless with good reason: cell phone ping records, gas receipts, CCTV.
Finally, WHY Yosemite in the last text supposedly from Gabby to her mother? I'm trying to figure out how that would fit with either "Gabby goes on alone" scenario OR as an attempt to obscure where she really was if she'd been abandoned.