as far as feeding goes, i would actually recommend feeding inside of the main enclosure. Less stress on the snake because they often prefer to hunt from the entrance to their hides. you can still have either of those substrates if you wish- i have never used those but have heard good things about each. Whichever one you prefer, go with it.
As far as the poo goes, my snakes usually don't stink too bad. Not that i recommend smelling it up close.
Yawning? normal up to a point. If it were to become a regular thing, i would be concerned.

hope that helps!