Ok so if you have a leak it might saturate the plywood between your inside membrane and the Styrofoam. Not saying you will get a leak or ever will just pointing out a major point to thin about.
But it would insulate very well.
Yes, it will be used on the edging which would be submerged to hide the liner. I don't want to use concrete to secure the stones so I'm hoping the regular foam spray will stick the stones to the liner.