Many factors play into this it is impossible to calculate
12" is the minimum distance from the wall
The boxes are not big at all. Two boxes, both are single person lift. They fit in the back of my 2004 Mercury Mountaineer/Ford Explorer with no problems. They would probably both fit in something as small as a Subaru Outback with no problem.
Definitely needs to be flush mounted. The filter housing opens on the top and is hinged at the back of the air handler. If it's inset, you won't be able to open the top panel to service the filter.
you really don't need a ground fault on the unit run a single line to each unit not sure why you would want to ground fault it
I would say no. The vents are on the bottom of the unit. Air flows out and at an angle. I feel like it wouldn’t function properly if placed low or near the floor. I would follow the guide included for placement.
it slows way down but it still runs to distribute heat or cool
We have noticed no significant increase in electric bill with this unit. I would say it'll probably cost less than a typical window unit to operate. Best bet is to compare the Energy Guide for the comparable systems. This is a DIY system so almost all of the installation can be done by a semi-proficient DIYer. However, you will probably need an electrician to set up the disconnect that's required by most electrical codes, and that can run a few hundred dollars.