Andrew, you've over-complicated something that's really very simple! And you're using a lot more of your fresh water than you need to.
All you needed to do was re-route the toilet intake line to tee into the head sink drain line below the waterline. This allows you to flush using sea water..then at the end of the weekend--or even at the end of each day--close the drain seacock, fill the sink with CLEAN fresh water, flush the toilet. Because the seacock is closed, the toilet will pull the water out of the sink, rinsing all the sea water out of the intake line, pump, channel in the rim of the bowl and the toilet discharge line.
Never use bleach in the sanitation system--it's destructive to the rubber parts in the toilet...and NEVER EVER combine bleach with vinegar...that'll create chlorine gas, which is lethal! A cupful--2 cups at most--of undiluted distilled white vinegar flushed down the toilet once a week before the boat will sit will prevent sea water mineral buildup in the lines...you can use the sink to do that if you want to. Follow with a clean fresh water rinse after 45-60 minutes. Never leave vinegar sitting bowl, 'cuz when soft rubber (the joker valve in the toilet) is allow to sit and soak in vinegar, it swells and distorts.
--Peggie