So.. here's our (now successful) insurance update!
Progressive: Progressive has told us they are canceling us on July 27, unless we purchase & insure another vehicle/scooter. They will not accept that we each carry a 'non-owners' liability policy with intentions of renting/borrowing vehicles when stopped for running local errands. Our agent was able to get them to drop the 50% 'multi family' premium increase for us being unmarried. In addition, Progressive will only cover our personal possessions up to $1000/each for a maximum of $13,000 (which doesn't help us much with our high end electronics, like laptops). And of course, Progressive only covers 'actual cash value' for older conversions.
GMAC: After much prodding from our agent, they have agreed to cover our bus for full-coverage (GMAC will typically only do liability for non-professional bus conversions), as we were able to provide them with our video tour and plenty of blog posts/photos showing the bus is a quality conversion. GMAC does not have penalties for non-married ownership, nor any requirements for owning a second vehicle. So this gives us more time to get comfortable driving the bus before rushing into buying something else. They'll also cover our personal possession for whatever level we wish to purchase at replacement cost, and coverage for the bus itself will be for market value inclusive of all the upgrades we add (solar, etc.).
Solution found, and we're super super happy with our agency and the extra work they went to find us a solution - http://www.epic-ins.com .