[Ram Montana High Country]

On the basement doors, folks have taken foamboard and cut two pieces that stack on the inside of the basement to block off the opening when the door is open, and the outside appearance does not change.

For the MINI-SPLIT, I'd talk with your resi furnace guy. I think he ground mount the exterior unit and run the lines inside. Ask him to look at alternate air handlers for the inside. He might be able to mount in the basement and have it force air thru the floor ducting.

If you don't do the mini split - do set the large LP tank and have a gas person tie it into trailer.

IMHO