So why would you install a macerator if you don't plan to dump the tank
at sea? Or do you think a whole tankful is better for the environment
than a single flush?

I don't want to install a macerator.

I am considering a new, simpler, and easier to maintain head
arrangement that would include 1) manual freshwater flushing, 2) no
macerator, 3) no Y valve, 4) Lavac or Skipper toilet, 5) just a
freshwater intake, a vent, and a deck pump-out, and 6) high-quality
hoses.

I am thinking about ordering a new Valiant 42. This set-up would
eliminate two holes in the boat, right? And, this would seem to be a
higher quality alternative to what you recommended a few strings up: a
permantent porta-potti with pump-out feature. Simple and easy to
maintain.

Just getting some ideas here. Thanks!