but would like to have the security of a Whaler in the shape of a flats skiff.
They already make that, it's called a carolina skiff
Your logic is sound and flawed at the same time, in perfect planned conditions you may be able to self rescue, but you may not know you have a bad leak until you are headed back to the dock in deeper water. Remember to plan for the worst and hope for the best.
Taking big waves over the bow is not a big deal to me, that's what I have the bilge pump for. I've had 4" of standing water, plus whatever was in the bilge and chases, in my skiff after taking a huge wake from an jacka$$ and wasn't worried. Pump kicks in and a few minutes later we kept fishing. But I know I can totally swamp my skiff and it will still float fine because of the amount of positive floatation I have in it.
There is something else to consider with large void spaces, without foam filling it you have a huge void of just air. If sealed, especially in warm climates, the air will expand and contract on hot/cold days and warp, crack, or rupture the surrounding areas over time. Filling the void eliminates the air space and will prevent this. Or you can add a weep hole, but that defeats the purpose of a sealed space, and if you plug it you have the same issue of expansion on warm days.
I didn't know all this until it was explained to me by Jacques from BBC when he was critiquing one of my designs.
I've used styrofoam sheets in the past with great results, but the closed cell pourable foam the have these days is way better then it used to be and I am quickly becoming a fan.