What I don't see is the need for the vented high loop between the toilet and the tank.
You need the high point loop!!!
Or Hold tank can run backwards to head. The bottom of the high loop hose
MUST be higher than the boat water line by a few inches.
The siphon break on high point is
NOT needed as long as the head fills your hold tank from the top, tank is
NOT full and the hold tank is vent is
NOT blocked (notice all 3 things
MUST be true. In essence the hold tank vent would break the siphon with perhaps the volume of the Head-Hold tank line flowing backwards for a bit.
The siphon break just prevents siphoning the hold tank to head and over flows in cabin.
My recommendation is put the high point siphon break and maintain it once a year.
If it was just grey soapy water, no big deal
But...
Otherwise .... not much fun on a cruise to clean up head back flow.
My boat is similar to kloudie1 diagram in post #44, except I have a macerator pump. Notice item #14 in his diagram is "vented loop". Hunter has years of design experience!
From my Engineer view... It works fine.
Here is worst case...
Seacock on pump to sea is open and pump fails (that happened to me but we rapidly closed the seacock, whew). Water fill the hold tank then will back flow thru the head and flood your boat if you high point is
NOT above boat water line.
It may be confusing, but there are several things going on.
The real solution to put a head on the deck with shower curtain.
Just kidding.
Jim...
PS: I did comment on your diagram #2 showing connecting the siphon break to vent in a closed loop. That won't work.