70 seems a bit cool. Most people keep their cool ends around 80. The hot side sounds good. It sounds like your setup is a nice one.

I've heard other people talk about "sneezes" and haven't ever heard of them turning into anything serious. Of course, that doesn't mean it couldn't. The impending shed makes me think it's benign, though. He's probably got some dried skin in his nostrils. I'd wait for him to shed out and see if it goes away.

Have you tried providing a humid hide on the warm side of the tank? He might like that to help get ready for this shed.