Since your friend was driving your car for an extended period of time, they would need to be listed as a driver on the policy. If they are not listed on the policy and something happens, the claim can be denied for not listing all drivers of the car.
Massachusetts is considered a No-Fault state and requires Personal Injury Protection coverage. This means that your insurance company will pay for your medical injuries no matter who is at fault in the accident.
Yes, typically it is cheaper to bundle your car and home insurance policies with one insurance company. Most companies will give you a multi-policy discount when you have more than one line of business with their company.
If the scratch was the result of vandalism and not caused by another vehicle then filing a comprehensive claim should have little to no impact on your future rates. This also means that you will have to pay your comprehensive coverage deductible for the repair.
The reason banks require comprehensive and collision coverage on a vehicle in storage is because it could be damaged by a fire or a collapse of the building in which it is stored. In addition to the bank, Massachusetts requires any vehicle registered in the state to also be insured.
While the state does not actively monitor drivers' compliance with insurance laws, Massachusetts does require that vehicles be insured while driven on the road. Compliance monitoring is done through traffic stops, after accidents, and at the time of registering or renewing the registration on a vehicle.
The decision to file a claim would be completely up to you, but in Massachusetts, an accident has an average impact of more than $700 per year in additional premium. If the repairs are less than that then I would recommend paying for the damage out of your own pocket.
When test driving a car from a private seller, the liability portion of your policy is what pays for damage you cause to someone else while driving so the fact that you don't have collision coverage won't keep the vehicle you hit from being repaired. As for the vehicle you were actually driving, you may have to file a claim with the car owner's insurance company since your policy does not include collision.