Partial Answers
OK,
1. The tank is much smaller than I had suspected. I don't have access to it without taking out some panels, but it appears to be 9" x 30" x ?" -- I can't yet see how deep it goes, will figure that out another day.
2. The tank was over filled and coming out the vent line after all.
3. Ship yard simply rendered the y valve locked in the holding tank direction with a screw in the handle to prevent movement.
4. The setup has a raw water intake thruhull, an overboard discharge line and a line to the holding tank -- y valve is mechanically set to only flush to tank, via a set screw in the lever of the y valve. Fair bit of rust staining and corrosion on the thruhull valves, as seen in photo below--will clean them up and reassess their viability, however it looks like they are fitted to the hull with plywood. After I clean up this compartment I'll be able to see if there are occasional water leaks. Everything looked dry at first glance.
5. Jabsco flushing mechanism, I think I saw some paperwork indicating the potty as a Raritan, but I'll have to confirm that later.
Might be easier to install a tank monitor than dismantle the cabinetry to place a light in the tank compartment to aid in seeing the fluid level.
Here are some photos for starters.
I have added vegetable oil in the bowl to lubricate the soft components, and have added some tank treatment (don't recall what it was, it was there from PO). That's all the maintenance I have done (to the head at least

) since purchase.
Does this look like a Raritan?