Don't see too many of them made.
For the love of god, don't do this. I ruined an assault rifle I pepped in my early days of armoring like that (as well as a few other things). Ahh, how foolish I was back then. But as for hardening it, Id highly recommend coating the outside in fiberglass resin first, to prevent any warping from the rondo and to give you a nice painting surface. Expanding foam does have its uses in armoring, but unless you really know hat you're doing, this is not one of them. Also, you're pepping looks great, keep it up!shooting it full of expanding foam (which didn't end well)
The way I did it for my Hayabusa build was first I hardened my MKVI chest piece, I then added the Hayabusa chest perm to the hardened MKVI chest while it was still paper, I taped the Busa perm where I wanted it to fit, then hardened it directly on top of the MKVI chest, once its dry, hardened it the normal way you harden your parts, you should be able to lift it off the chest piece to get to the backside of the perm for hardening, If this is all hard to understand just look on my busa build in my signature, there should be some pics on how I did the chest sections, hope this helps
Yes, this is probably your best bet. Depending on your build, you might also want to consider making lifts on the shoes you are wearing, Using Shoo goo and EVA foam can give you a much needed lift, and filling out your body suit can make the parts fit more desirably. Judging by your body build, you might want to consider filling out the chest area and legs a bit more to give it that armor-lift look.