5. Reinstall – what provisions/precautions should I take? Set base on neoprene, provide neoprene “insulating” material between structural elements and tank shell. Anything else?
I would coat the exterior bottom with a polyurethane, such as Loctite's Black PL S30
For a more uniform coat, cut yourself a notched plastic trowel, put on some ribbons, let it cure a day, then go over that with a smooth trowel. This stuff will stick well to aluminum, and is 100% waterproof.
If you want the rubber super tough, mix in 30 to 40% 1/32 fiberglass milled fibers, turns into a rubber like on a car tire.
Loctite PL S30 Polyurethane Roof & Flashing Sealant from Loctite Adhesives
This stuff also works underwater.