That was me. Good Questions.
First off, WD40 displaces moisture, so that is the good news. Whatever product or procedure you use, just get the corrosion off the connectors.
If it were my car and the connectors were heavily corroded, I'd probably pull the fuses in groups, after carefully noting what rating fuse goes in each spot, and clean the brass holders with say a fine sandpaper, steel wool or brush. I would then excessively vacuum all the debris out.
After that I would spray WD40 onto a rag and wipe each connector liberally, erring to the side of too much. Probably spraying a bit into the box generally will not hurt as long as it isn't excessive.
I would then check each end of the fuse to make sure they are clean as they are inserted, but before doing so, wipe with a very lightly WD40 saturated rag.
Once all this is done I would rotate each fuse in it's holder to ensure good contact.
I guess all this is a bit excessive but I have done this before when living in humid climates and it works well.
One other alternative to the aluminum type fuses is to replace them all with the brass type. This doesn't eliminate corrosion but they seem to remain more consistent over the years. Often you can find them on ebay.
Do you know why moisture is getting under your fuse cover? Have you checked the body drains close to the fuse box to make sure they are draining properly? There are typically two, one on each side of the car. It is very important that they stay free and clear otherwise you get water build-up after a rain or carwash.
Hope this helps and may your fuses stay shiny,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)