From what you say, it sounds like your mic was "open" and the set simply transmitting. I'm guessing that that might also prevent the set from changing channels. I seem to recall someone else on here having a similar problem and b1tching 'cos the mic was hardwired to the set
If it's corrosion inside the mic, you're stuffed. I'd try rinsing the outside of the mic in case it's a build up of salt on the controls holding a button in. A longshot but free to try. A couple of slaps wouldn't go amiss either
My experience so far (owning a 7100) and seeing several others is that they are prone to corrosion on buttons and knobs.