It's not he said she said though considering he drove to the middle of nowhere. Despite the common joke, cops aren't (usually) stupid. Op's ex would have to explain 1. Why he skipped work. 2. Where they were going (his store excuse won't work since he drove past stores and the timing is suspicious).

1. Didn't want to go in to work (doesn't have to justify leave used)
   
2. Going to a store in XYZ that's along the route (doesn't have to legally justify not going to other stores)
   
3. Wanted to get it done early because I wanted to use my day off to do XYZ (doesn't have to justify timing)
   

It will be very suspicious, but as you said, "honestly neither one of them will probably be charged with assault."

Also defending yourself amidst an attack isn't assault.

Self-defense relies on proportional force and presumption of harm. If he is physically removing her, and she is fighting to remain in the car, it's a complicated question of how much danger could she reasonably be in? At the same time, he's picking her up, she's kicking him with "all her might" - so is the use of force proportionate?

What will happen is there is an official record of what happened through the police report.

And this is why it's important for her to see an attorney before the record is made - so it can be as clear-cut as possible and be used in any future litigation/proceedings. If she just runs to the police, she's going to make gaps that a good attorney will be able to pick apart later on.