A y-valve won't let the black water
flow from the tank to the overboard
discharge. It will just send it back to the toilet. You'd need to make a manifold of two or more valves to do that.
I would suggest you simplify the system by having the toilet pump directly into the top of the tank and mount a discharge fitting low on the tank. Put a Tee on the discharge fitting with one leg going to the seacock and the other to the deck fitting. The tank of course needs to be above the waterline to gravity drain. It would be great if the toilet was higher than the tank so the Stuff always flows down hill.
A valve on the tank discharge before the Tee is a good idea.
It's probably not a good idea to pressurize the tank with an air pump. I'd worry about blowing out a hose connection. You'd have to close the tank vent to do it anyway.
Like Ron said, but Peggie Hall's book.