Ah... Evidently you didn't notice/understand the symbology. In the given illustration there is no "macerator pump". There is only a hand operated Whale Gusher diaphragm pump to direct/pump waste from the holding tank to the through hull. With a good enzyme "digester" additive to the holding tank, the Whale Gusher is quite good at moving the liquified effluent out of the boat. This arrangement worked well/reliably on our former yacht from 1976 until early 2002 and has been working equally well on our current yacht since 2002 through the present. But heck. What do I know, eh?It's actually a rather poor diagram.
The macerator should have its own thru hull, because the outlet of the macerator is much smaller than the head outlet.
Therefore, that Y valve isn't needed.
The pump out should have its own connection to the tank.
So, only one Y valve is needed on the head outlet: overboard or to the tank. And you could do it with a simpler ball valve.
This discussion about vented loops is wrong, unless I'm reading it wrong. It's simple: vented loop between the pump on the head and the head bowl. It's a 3/4 inch line with a 3/4" vented loop, nothing to "carefully size" at all. This is required basic stuff and available in ALL head manufacturer's literature for head installation. The heads come with a short piece of white hose between the pump and the bowl. Replace it.