If fact, one can over-tighten the bolts holding a joker-valve in place and, if the flange for the collar is not perfectly flat, that can spread the lips enough to allow leakage, particularly it the water column is not great enough that the water pressure forces them closed. It really doesn't reguire much torque on the bolts to create a water-tight seal around the collar base.
Good advice on how much to tighten those bolts.
I have owned boats with marine heads for almost three decades. No "joker" valve ever leaked back regularly
. Once in a while we can tell that a tiny piece of paper is hung up in the valve though, by the slow back filling at the bottom of the bowl.
Lots of folks turn off the head discharge thru-hull when not using the head; we turn it off only when leaving the boat.
(None of our boats ever has had a vented loop on the outflow... but that does not mean it is a bad idea.)