Kirsten, (you've been busy on the computer today, eh? ), what I know about metal stains has been mostly learned here, so I don't have much 'practical' experience with them.

#1 - the pool water shouldn't come in contact with the wall panels, but I know that it sometimes does. Using an epoxy putty can seal the joint which is allowing the water to get to the steel.

#2 - On the Perflex filters, the best way I can think of is to turn off the pump, plug the return(s), open the drain plug, remove the DE, and attach a hose to the drain opening going back into the pool. This is as close to 'bypassing' the filter nest as I've been able to come up with, and I won't guarantee that the fingers in the nest won't be exposed to the water . Or, you could completely take the filter out of the system - take the union apart, unscrew the adapter on the top of the pump and hook up a hose going back to the pool (**if you don't know jack about plumbing PVC, this would probably be a real PITA - but there are people here who could 'walk you through it'**)

Sorry to be a bearer of bad news today but perhaps something I've said in these 2 posts will inspire someone else to a better answer