For what its worth I'd go for something based on your "With sheets of Divinycell, I was thinking of gluing together 3 sheets of 10mm to form rocker, cut outline and then shape some bevels on the bottom rails. Otherwise flat bottom and deck"
For a general freeride board, once your flying I don't think the board shape matters too much. Obviously a wide nose with plenty of scoop helps with touchdowns and sharp edges should be avoided. Maybe have a look at the Ketos boards for some inspiration?
For the core I'd go for the H60 or possibly H80 for improved crush resistance.
A while back I built a board similar based on your suggestion but using only 2 layers of 10mm foam (75kg/m3) http://www.easycomposites.co.uk/#!/core ... -core.html
the layup is describe here http://kiteforum.com/viewtopic.php?f=19 ... 95#p956395
This board is still going strong but I have decided it is just too short/light, for me....combined with a light foil (Ketos) I think the lack of inertia makes for twitchy ride for an old man with slow reactions. I'm fine on the same foil on a longer heavier board.
So, I'm now in the process of making another (135 x 48). The main differences are:
1) 1 layer of 200gsm glass rather than two.
2) Carbon patches for plate mount reinforcement now 1000gsm ! and about 50cm long on top, 40cm long on bottom.
3) Core is from one 25mm sheet with nose scoop built in using kerfing.
The lams are complete and now it just needs a filler coat, sanding and painting. Currently just under 2.5kg.
With both this board (and the last) I can support the board at each end and confidently bounce around in the middle, with the board either way up...Not a very scientific test but it does give some confidence.
Finally I suspect that with most foils/boards a real hard grounding will result is something giving, be it the foil, board or you! Given that you have the skills to build/fix a board, it seems that this would be the best bit to 'give'
Anyhow good luck, whichever way you go.