Thanks for the reply on the cooler box assembly, I'd be interested to see an under dash picture when you have the vehicle back again.
On the tire note, I think a spare tire on or off the rim takes up about the same space, so I'd have it mounted on the rim, although off the rim would be lighter.
Roof mounted would be a PIA to load/unload. I like the under-car spot, but you'll have a water tank there...have you thought of mounting it in the front, off the grill area? Might be in the way of a winch.
I've put 7-plugs in a sidewall, aired up w/ CO2 and made it back to town, just couldn't park with the split on the bottom of the tire revolution or it would split open and start losing air again.