Are your UTH's on a thermostat or a dimmer?
Sand? Since you already seem to know that it is not recommended - I will just walk away from that one
I don't think it is necessary to put a florescent light in, but again that is just my opinion.
The light that you are currently using to up the ambient temps - how many watts is it? Maybe you could put in a red or blue light with less wattage to help with your ambient temps. Ambient temps IMO should be close to 80 - 85 degrees. 100 degrees is excessive.