You might want to find a plumber who can answer your question. Or a
competent inspector, who will have to check your work anyway (right?).
Venting can be tricky, and if you are putting this in concrete, then
you want it done right.
As an aside, I think you certainly do need some venting. Imagine a rush
of water coming down the stack, past your whole layout here. It would
tend to pull air out of ALL of the traps, and the toilet. There are
rules about how close the vent needs to be to the ptrap -- having to do
with how much elevation is lost between the ptrap and the vent, and the
pipe diameter. I would guess that perhaps the 2" vent in C would
probably work, or at least, it seams at least plausible that it might
be at least part way to being what you need, most of the time.
Depending. And YMMV. To meet code, you would probably need at least two
vents, much closer to both the tub and shower.