When test driving a car, who is responsible for any underinsured expenses/coverage, the dealer's insurance or mine?

The way it would work here would be you take the drunk's policy limits only w/the written permission of both the dealer and your own UIM carriers. Once you have it in writing, I would think the dealer would be on the hook first as insurance goes w/the car, and if you exhaust that policy, your own policy might kick in....I do not know how offsets work in IN, if at all, but that will be another possible issue. Speedy recovery!

You insurance will cover your medical bills if you have personal injury protection coverage, and even then only up to the limits of your coverage. The dealership probably has insurance as well. Have you looked into that? You may have to bring a claim against the driver and the dealership for pain and suffering as well as any unpaid medical bills.

That is a really good question. Typically the coverage on the vehicle is primary. However, most if not all auto insurance policies contain what are called "other insurance" provisions, which address which coverage is primary, excess or pro rata in the event that multiple coverages apply. Underinsured motorists claims can be tricky and I would suggest you consult with an attorney in your area for advice. Good luck.