Book value is used when the insurance company decides to repair or total out your vehicle. When the cost of repair exceeds a certain percentage of the replacement value, they won't fix it but rather they'll cut you a check for whatever the replacement value is.
Nobody in the business uses Kelly. Banks typically use NADA values. Dealers use black book - which my understanding is now an online service where they can see what the wholesale prices that actually get paid for cars at dealer auctions.
Repairs don't typically add anything to the value of your car - the value is based upon the vehicle being in decent mechanical condition. The premiums are based upon the vehicles being in stock condition. If you add a lot of expensive extras, they may or may not be covered if you don't disclose them.
I would find a new insurance company. If they're using trade in value, you're screwed no matter what vehicle you have.
I've got USAA, and I've never had a problem with vehicle valuations.
Speaking of shovels, agreed value policies don't necessarily limit the mileage or use of a car. The classic car license plates/registration from the State may place a limit on the annual mileage in exchange for reduced or eliminated registration fees. If you don't have classic car plates, that's not a concern.