Niiiice board.
Those channels moulded or shaped?

no probs with model CAD/CAM its my workingday.
Machined concave yes but rocker it is just a waste of material IMHO.

what kind of model do you need, it can be done in different ways. Easiest way is to mill flat core and than laminate it on a rocker table.
http://www.youtube.com/watch?v=uwPuSg-xYHs

4 1/2 for a fish? I think it's a fish no more.
A while ago i red on Sways that big surf rocker is too slow for fast kitesurfing. So i think it depend from local conditions.

Balsa density varies from 150-320 kg/m3. It depends where this blank was originally located in the log.

Not nesesary. you can add them in lenght.
http://www.blacksail.fr/index.php?rub=M ... 0a%20press
see link above.

I would sugest you to make mold out of original and than make copies, cheaper, but with little quaity loss. TFC is makeing theif fins out of G10 this is heavy stuff for a tool (router bit).

I can do not only CAD/CAM but also have an possibility to machine it.

Goood board wery nice.
This green stuff (board rails) what material is it?

You can seal the crack, but if the core is destroyed (or damaged) all the pressure will concentrate in that cracked aria and it will lead to full destruction. You also have to fix core, replace damaged part, or compensate it with laminate. But the board will never be the same again. Probably repair ...