I'd push the pin on the handle gear then turn handle and see if clear then do the next to find which one is tougher. My guess is is needing lube in the vertical shaft but unsure how.
Cabinet screws all the way. Besides the washer head the threads are different than a regular screw. Do some googling to find what you need for what you are doing.
Sounds like the tank is not venting. Try running with the cap off. Im assuming it is a portable tank and you have already ensured the vent screw on the cap, if it has one, is open.
I know this setup is way diff than mine but I could not identify leak location(s) by filling up the boat. The sand silt and muck plugged the holes. Until I ripped the floor up and cleaned out no water dripped. Do it anyway just thought id chime in on my experience.
