Sorry, I do not see a valid basis for a lawsuit. They ill consequence you face is a rate increase. Insurance company evaluations of liability are not issue determinative. In other words, they have no value in a court of law. Ask to speak with a supervisor.
If you believe your case is being mishandled, you may have a claim for bad faith against your own insurance company. I can't determine if such claim exists based on these facts, so you should contact an Avvo rated attorney for help on this matter.