Salt as I’m sure you know is no good on anything except maybe certain foods. The DOT seldom use it in Oregon but we do trips to the coast and dunes and it gets exposed. I built a spray bar that attaches to my pressure washer, it has nozzles every 8” and is long enough to get the underside of the Jeep and my RV, simply hook it up and slowly drive over to rinse the underside. Back to the black fenders oxidizing...door handles and hood latches as well... you can always use a heat gun to restore the plastic...there’s a few videos on how to do this.. the creams and sprays make it shiny and new looking but wash off and the fenders go gray again..just remember to tape off the painted surfaces and take your time with a heat gun in a steady circular motion, you don’t want to melt it and the plastic must be thoroughly cleaned first.The short answer is yes, but the sun is the bigger contributor to fading. Heat, cold, dirt, salt will all contribute to fading so a regular application of some sort of protectant is a good idea.
Yes, when to far gone, just replace with aftermarket fenders or buy new OEM black plastic fenders from a Mopar parts vendor.They are going to get scratched up anyways on the trail. Better to scratch the black plastic rather than the painted fenders. Then just replace them down the road.