All of my snakes poop when they shed, I don't think I've ever had one shed that hasn't pooped at the same time. I think that's totally normal. Personally I wouldn't use an overhead lamp, just dries stuff out a bit too much. If you have an undertank heater I'd probably turn off the light just before a shed and crank up the humidity. With a coconut substrate like Reptichip you can really soak the substrate and it will still be fine, it won't mold at all. I spray my substrate down until the coconut is dark brown, just before it really gets drenched, just nice and wet. And in 2-3 days it dries out quite a bit so I'm always spraying it down with a lot of water. If you are having mold issues I would certainly change to a different substrate.