Our trip will begin (and end) with a 10 hour drive to Tampa where our cruise departs. We may break that trip up and spend the night en route. Will a comprehensive policy cover us in the event the car breaks down along the way?
No it will not. Any mechanical problem with your own vehicle would not be covered with any policy.
I have a nonrefundable roundtrip air ticket. If I need to change the return flight, the airline will give me a credit toward another flight, minus a $200 change fee. Therefore, although I must insure the entire cost of my roundtrip ticket, my actual loss will only be $200 (plus additional cost of the new ticket, if any). Will trip interruption coverage at 150% reimburse me $300 for the change fee?
It would depend on the plan, since they would all be worded differently. But usually, when there is 150% Trip Interruption, they reimburse for the unused portion of your prepaid, non-refundable trip costs and reimburse for the additional transportation expenses incurred to reach the return destination due to one of the covered reasons. You can click on the amount next to Trip Interruption when viewing a plan to read the details of that specific plan.