Sorry for the delay in responding. We contacted both companies that we represent (HDI and Chubb) and were waiting to get their direct feedback on these questions.
HDI will cover theft when a vehicle is off road. HDI will also cover losses due to fire, so long as the fire was not caused as a result of being off road. (Example: If a car overheats and catches fire due to off roading in a sand wash, it would not be covered. But if a car is parked off road and gets hit by lightning and burns to the ground, it would be covered since the loss was not caused by being off road). Chubb specifically excludes coverage for any type of loss (collision, theft or fire) that happens off road.