The head is finally finished and can be used. I installed a longer discharge hose so it can be looped above the water line.
This way we can get rid of the third ball-valve the previous owner installed to prevent the water flowback to the blowl while heeling. I also replaced the hose from the seacock to the waste tank.
The Jabsco toilet should have a built-in anti flowback valve, but I've read that it usually starts to leak, so just to be on the safe side I also added an extra blackwater anti-flowback valve close to the toilet outlet. If it will cause problems in the future, it's not that difficult to remove it and replace the discharge hose with an even a slightly longer one. The cabinet and the lining inside still needs a good cleaning, and I'm planning to redo all the bottom cabinet shelfs too at some point (after we replaced all the water hoses, so they'll match the new arrangement), but now other things have higher priority.