Really? Where did you read that? Surely they can't just keep it on if they can't be bothered to shed? :S Not disputing you though as I don't know any opposing evidence.
(try to put it on the heatmat since it will evaporate some of the water and really raise humidity)...
I think you have to be careful not to put it entirely on the heatmat, it can just get REALLY hot inside. I've read a horrid story on here where that very thing happened. I think the point of a humidity box is to put damp paper towels or moss or similar inside and it stays wet and warm so the snake pulls of their own shed and it's moist enough to do so.
Once the digestion of the old scale layer is done and the old one is thin enough the new one grew up enough, the liquid is absorbed back by the skin body and then its in the snakes hands(so to speak since they don't have hands) to remove the old scale layer. in fact not every snake sheds at the same number of days after they're out of the blue. Of corse its in their instinct to remove the old skin once its ready like it is to eat and poop. i don't know if it bothers them or not but i guess if i had another layer of skin on me while moving that its loose from the other layer of skin it would bother me.