I've been running a dash cam since I got the new van.  It's powered by an auxiliary circuit that powers up the camera any time a door is opened or whenever the vehicle is running and then stays on for 30 minutes after any activity in the vehicle.

It has GPS tracking along with the video and while they might say you erased the video of the accident, the GPS tracking may have been enough to prove that you were in another area and not near the accident scene.

I don't have any experience with this kind of thing, but curious why your insurance company isn't more involved in either fighting or settling this.  The few times I've needed my auto insurance, it's been really hands off for me.