I have some Aqualung sets which have teh Auto Closure Device (ADC) that closes the 1st stage when you remove it from a cylinder. My regs get fully immersed overnight in my rinse bucket after diving (most weekends) and no damage. My repair centre would tell me at service time if there was any damage.
Regarding discoloration of the threads on a Din set. all mine have done so. any coating as new will wear with the constant screwing on an off. as long as teh Din screws in easily and the sealing face is clean then you should have no issues.
People do suggest you can clean the threads with vinegar, but you'd need to ensure none goes' in the 1st stage. I've never tried it, so can't recommend it.