Not sure what the original construction was or whether you potted the magnets. I don't trust the nickel coating alone under these conditions. The steel/nickel interface is even more of a problem.

I am inclined to think that for this sort of environment galvanising of the steel discs is the way to go.

Regarding the other factors I agree with DaveB and I agree with Ron that superglue is one of the things best forgotten, It sticks paper but other things do as well. I see no use for it in this field.

I believe Hugh has had some luck with some form of bitumen/epoxy paint and that may be a way of keeping the problem in check with what you have got to deal with.

If you can satisfactorily protect the magnets without completely potting them I go along with it but potting may give better protection in the long run under these conditions. If you pot with satisfactory material I see no need for the stainless band round the outside but that is an individual choice. My concern is corrosion not magnets falling off.