Whats the inspection consist of? Just a quick visual between the seats to see if the seat belt is bolted to the seat or is it more extensive than that?
I think I used .045 or 1/16". I recommend using stainless so it doesn't rust. If you have a welding supply store near by just stop in and ask for a piece of .045 or 1/16 stainless filler metal.
Fennel coating seems to be the only thing the works around here (PA). I use the quart cans and an undercoating gun. http://ducarle.com/ I use fluid film for the inside of the frame rails and body panels with the wand kits. The fluid film will usually just get washed off the outside of frame where as the fennel coating sticks like glue. I touch up yearly although with the fennel there's not much to touch up usually.
Yes they can walk around freely, and it has worked out great. When you need the cover off for people you can just unhook it from the headrests and fold everything under the seat. I have another picture showing how it attached to the bottom of the seat frame but the file size is too large. If you dm me your email I can send that to you as well.
I use and really like the 4knines hammock seat cover. Instead of having the hammock part attach to the front seats I tuck it under and clip it to the bottom of the seat so that can still fold the seat up when needed. Doing that makes it much easier to vaccum out the dog hair weekly and also keeps the dog hair from sticking to carpet under the seat. I've had it for a year and so far is holding up great.
For the front seats i use the carhart seat covers.
https://4knines.com/collections/the-best-pet-car-seat-covers-around/products/dog-seat-cover-rear-bench-seat-for-cars-trucks-and-suvs