what kind of bedding? what's the humidity in your house, outside of the tank?

You are correct in measuring the temp on the glass directly over the UTH. If they need more heat than they are getting through the substrate they WILL dig...

Also, for the humidity, try a smaller diameter bowl. The depth of the pool is irrelevant. Evaporation occurs at the surface, and the more surface area you have the more evaporation can occur. Think about it this way... if you set a glass of water outside next to a large bowl with the same amount of water in it, which one would evaporate first? The bowl, because it spreads the water out more...