Sorry for your problem. Last year a friend of mine was mirrored by a car (hit and run). Drivers in front of and behind offending driver stopped, assisted cyclist and provided police with id. Riders over 10 year old Ti bike was damaged but wheels and groupo were still good. Ti frame was cracked. Driver claimed he did not do anything and refused to pay for bike. Driver convicted of traffic violation including leaving the scene, then cyclist filed claim with drivers insurance company. Company originally offered $300 without adjuster seeing bike. This was refused. Adjuster came, saw bike, reviewed replacement estimate prepared by LBS and provided sufficient $$$ for a replacement bike within 3 weeks. They did take the old bike (everything on it) but he was made whole. They also asked him to sign a waiver stating any future pain would not be paid for by this claim. Completely different result than OP is getting. Only difference I can see is traffic violation upheld.

I have no advice for you, clearly this ins co had decided to play hardball by lowering the offer. I'll ask my friend for more details on how he handled, maybe there is something missing from your strategy. I do know he continued to ask to be made whole and that is ultimately what happened.