I've never seen the mouth thing but I have had a very similar experience to yours. When Snakey sheds he camps out in his water dish solid for like a week. He looks absolutely pitiful.

So, I have his water dish on the warm side and partly covered so he can safely soak and feel relatively secure. When he does his big soak-a-thons I pretty much leave him alone but check on him regularly.

This last shed he would say in the water an not move AT ALL and have his head submerged to where only his nostrils were above the water. One day I peeked in on him and his head was laying sideways. It looked like at least one of his nostrils was above the water but I had never seen him with his head sideways before. I was cool but starting to get a bit nervous. I called out his name and tapped on the glass - nothing. Little more nervous. I opened the top of his cage and lifted the cover off of his dish. He didn't move AT ALL and he always starts to tense up a bit when I open the top. I was really scared then. I called his name and reached in an gently touched his body with my finger and BAM! he suddenly tenses up and contracts in the dish. He was clearly sleeping and go startled. He didn't strike or anything but he scared the CR*P out of me!

He turned out to be fine.