Regarding your first bow. If you have enough length for your draw length then you can pike the bow an inch off each limb and flip the tips. Depending on the bow, I would think you should be able to salvage whatever weight was lost on that back.
That said, what you tried to do with the linen and poly resin would probably work if you used a good ( smooth on or gflex type not 2 ton style) epoxy resin. I would not paint a layer over the back and build up epoxy as you did with the poly. I believe for good adhesion you’lol need a rough surface on the back like 50 or 60 grit sanded. Personally, I would pike it, toast the heck out of the belly, flip the tips and see what you can get away with, but I like selfbows. If it don’t HAVE to have backing I don’t want it to.