How are you reading the surface temp of the hotspot over the uth?

Having the substrate as thick as it appears will insulate and trap alot of heat. If the uth is providing a hotspot of 90 with over an inch of eco earth over it the temp at the glass tank bottom is going to be dangerously hot.

If your creating a 90 hotspot at the glass bottom and then putting the substrate over it the actual surface temp on to of the substrate will not be close to 90.

Either way I don't think your snake is getting much good from the uth. Unplug it until you get it sorted out. Sounds like your setup will be just fine with just the light so I would save up and buy a good tstat when you can afford it then start using the uth again.