Jeff, I will start by saying I cam not a chemist but putting a magnet in the fuel supply line will do nothing to alter the diesel fuel and you would just be wasting $200, it's snake oil
technology. Instead if you are starting with clean tanks
and good fuel just dose the fuel with one of the many bio products to stop anything growing in the water
fuel interface. Next keep the tank full to stop condensation
forming water in the tank and make sure the fuel filler has a good o ring seal.
The Griffin looks better but again comes with a serious price
tag. Why not just fit a single
water separator element?
Not sure you will need an additional fuel pump
why can't the engine driven pump
pull through a pre and then engine fuel filter which is what most boats do.
The bleed pump will be a lever on the engine fuel pump, but they are hard work
and you need to ensure the engine is "off the cam" or you will be tickling the lever for hours to no effect. Interestingly Peugeot cars in Europe
fit a little outboard fuel tank
bulb to help bleed the fuel lines. Fitting one of these would make bleeding the fuel supply line so much easier.