Yes, you can turn on and off each channel independently. I run one of my Herpstat 6 with only 3 channels on and another one with 5 channels on.
For sure. 68 is WAY too cool. I wouldnt go below 70F ever and thats only for an hour or so. For regular cage temps, i wouldnt go below 76F. I keep all my snakes with a 77-79F cool side and a 86-88F hot spot.
If ambient is cold, you might need to get a small CHE and put it on a lamp dimmer to get the ambient in the cage up to around 77-79F. A UTH isnt going to heat the air even close enough to go from 68-78F.