Your contractor is right about the vapor barrier in the crawl. Remove any debris, put 6 mil poly on the floor, lap seams a minimum of 6", extend the poly 6" up the walls and and columns. Now someone has to crawl over that fragile substrate in a tyvek suit dragging a 3" hose for the spray rig and a 1/2" line for his respirator. The variable you are not considering is the foam rig; a heated rig using foam components in 55 gallon drums will provide faster application, more consistent yields and a lower cost compared to the smaller 5 gallon component room temperature rigs. If your crawl is more than two feet high it will consume more than one set of room temp components adding down time to the job and increasing the cost. $6500 sounds about right for room temp foam. If you need the attic space for storage it makes sense to foam it, otherwise cellulose is a great value. At depths of about 16" it has comparable airsealing properties to open cell foam and it can be applied over whatever is already up there, and you get to retain whatever that original R-value was. Cells in the attic will be about half the cost of foam. As for your convection, conduction, radiation question, that's building science not building code. The code is prescriptive. It says you need R-19, you need R-19 regardless of what insulation you use to get it. 2" of closed cell foam is approximately R-7 and easily outperforms 5" of fiberglass at R-19. In an existing structure you get to do what you want, you only need to follow the prescriptive code if it is new, or substantially new, construction. Solar is an entirely different topic.