Hi Tim, bad luck with the van mate, sounds like a proper nightmare

Yes I have done mine with protectakote, floor and sides up to wheel arch boxing height. It is just about 3 years old now. I've re-coated the floor once in that time just as maintenance, hadn't worn off or anything, and it has held up very well indeed. Non-slip too when damp. Pleased with the way it has lasted. Absolute COW of a substance to work with though, get it on your skin and you have to wait for your skin to shed, it doesn't come off!! One other thing - Xylene is the thinning/cleaning solvent to use with this stuff. Xylene is carcinogenic so use long rubber gloves, respirator and don't get it on your skin. Cancer isn't fun.
I painted the rest of my ply exactly as you said - undercoat and gloss, one coat of each. Keeps it all nice and tidy, being a wipe-clean surface. The other thing is with white paint all inside the van it is VERY light in there, you're not fumbling round in the dark. I did what you plan to do as well, pull back the ply and stuff it with fibreglass loft insulation. Works an absolute charm, needs very minimal heating in winter. I extended the ply right up to the top of the side panels and across the roof too, as well as doing all the doors. It's like a giant cool box

Get a couple of those packs of different types of storage hooks, wickes or B&Q sell them. You have to mess around a bit and play with them to get it right but it's time well spent.
If you want me to send you some photos of the way I've mounted wands etc let me know your email address

Unless you can't get an electric lead to your van overnight I would forget the webasto/eberspacher idea. Why make another problem for yourself with condensation and a flat van battery every morning? Also there's the noise to consider.
I would also say that the checkerplate is not a good idea. No insu;lation properties, will be slippy when wet and damned heavy!