We generally recommend basing your liability needs on your assets. From the information you've given, I would say that maxing out your liability limits and getting an umbrella policy is probably a wise decision since you own your home and vehicles and have a high net worth.
Auto insurance will not get involved in this situation unless you were injured in a car accident or something else that would qualify as a loss. If your friend is at-fault for this happening, you could file a claim against the personal liability coverage of their renters or homeowners policy.
I have not heard of a claim decision being reversed and asking for the claims payout money back. They should be providing you with evidence of why they are refusing to pay the claim — if you haven't asked for this already, you should demand it.
A non-owner's policy will have a signed endorsement on it stating the insurer will not cover any claim that results from a vehicle the named insured owns or is registered to. Many will also state if it is part of the household, they will not cover it.