Hey, just finished reading through your thread. Looks good so far.
For the foam, if you are using 1/2" plywood for your deck, you'll want to make sure the foam is snug against the bottom of the deck to provide a bit of support, otherwise it may bend as you walk on it. It will feel much more firm with the foam providing some support. On my boat, I used 1/2" plywood and it would flex when stepped on, I added pour foam under deck and it firmed right up.
For cutting the foam I have seen others use a hot knife. As far as I know there is no easy way to get the pieces cut to fit. Let us know how you do it.
Keep up the good work!