Put the lamp (or CHE) on the "cool" side of the enclosure. Set it to maintain around 80 degrees. Then the UTH on the other side creates the warmer zone for the snake to choose when it wishes.

What sort of substrate are you using? If you've put in the coconut husk and its still wet, then the humidity will read very high...but will drop eventually as the stuff dries out. If you're using a dry substrate already....then adding a lamp may very well be helpful to you.

I wouldn't worry about the warmth causing bacteria growth in the bowl. You should be providing fresh water frequently anyhow.

You need to find a Home Depot or Lowes for the rheostat. I don't usually find them at WalMart.