Nice schematic, seems to work OK. My forward Lavac head setup has a Hendersen direct discharge pump and separate Whale emergency bilge pump - located further aft. The Y-valve therefore, joins the manual pumpout hose with tank & deck pumpout hoses.
There is another Y-valve accessible from the head, which diverts discharge from another Hendersen pump to either the tank, or directly overboard. This setup of course, requires two through-hull valves.
I actually did something similar as your setup when installing a new shower sump box. I connected a Y-valve between the Whale manual bilge pump discharge hose and shower sump discharge hoses.
True Blue . . .
sold the Nauticat