For #1.
All of my snakes yawn when starting their roaming at night. I think it's just a general stretching of the muscles, just like you or me. Except they dont have arms or legs to stretch lol. If she/he is sitting with their head elevated and is making popping noises etc, then that is when you should worry.

#2
Live rats can scratch. The only animals I have been able to see these marks on are my albino and the orange ghost. The ghost actually has a pretty little cut on top of her head that's healing up nicely. You generally wont see it on normals, or dark animals. That's why it's important to supervise all feedings, but it seems you got that down. I wouldnt bother cutting the rats nails, as I have a hard time doing it with my pet rats lol. Probably not worth it.
Just my .02$.