You can totally just remove the sand as you are thinking to start you BB tank. Its of course ideal to cook the rock before starting, but the same can be said for a tank with sand.

The bb will just make it easier to clean.
I wouldnt use the fuge area for anything. Just let it be a settling area with nothing in it so any detritus that accumulates there can easily be vacuumed out at water change time. If you put rock in there it will just make it harder to clean, will collect more crap that will rot, and you shouldnt need that extra rock for
biological filtration anyway.
The G3 will suffice, just as it has on your current tank, but you could do a lot better regardless of your substrate choice.
Assuming you are just using the return pump for just the return, and not powering any
canister filters or chillers with it as well, there is no reason to change to a larger pump. No reason to send any more water by your skimmer than it can process.
hth