Driver backed into my already open door and lied about it to insurance

I was returning to my car in a parking lot after running some errands. I open the driver door all the way into the empty parking spot to my left, to let in some air since it was a hot day. Then I start my car (still parked) while the door is still open. Around 6 seconds later, the other driver backs into the parking spot, hitting my open door, which has been open for at least 10 seconds at this point.
I received info from insurance that they ruled the responsibility as 50/50, saying I should've been more aware of my surroundings. I later learned that the other person lied saying I was opening the door just as they were backing in.

Thing is, I have a dashcam that started recording once I started the car. So there's 6 seconds of nothing, followed by the crash (there is audio and you can see the car shake). I submitted that to insurance but they said the video was "inconclusive" as it only showed what's in front of the windshield and didn't have the door/other car in view. If I'd opened the door as they were backing in it would've been caught by the dashcam, and 6 seconds is enough for someone to react to an open door.
I tried looking to see if there was an audio difference/lights turning on or off to show the door was already open, but I couldn't find any solid evidence in the video. Is there some other way to prove that the door was already open and that the other driver wasn't paying attention?

I am in WA state if that's important.