You probably need to keep the substrate a lot thinner where the UTH is. Substrate is a great insulator, which means the glass underneath can get very hot while the surface of the substrate isn't that warm. All else being equal (meaning, I'm not talking about what thermostats do - although you should have one), a UTH that is insulated will get a whole lot hotter than an identical one that is not insulated. It's like putting a pan lid down over the electric burner; even if the current is low, the burner can still get way hotter than it ever would even on high heat with no pan lid over it.
Which brings me to the UTH on the side. It may not be doing a whole lot, but it probably is actually doing a little bit. Whether it's enough to make a difference and whether you actually need it are other matters. But a UTH stuck to the side is basically a very low wattage RHP. It isn't likely to get excessively hot because it is exposed to the air on both sides, basically (the glass doesn't count as insulation). It will make the glass warm to the touch, which will in turn warm the air a little, and the IR it gives off will warm nearby objects a little. But it's still only a little, because it's a very low-powered device.
It still needs a thermostat though just in case. But a UTH stuck on the side is way less likely to overheat than one covered by substrate on top and sitting on a wooden shelf underneath.
As to whether it's actually useful, you can find out by comparing what happens when it's on and when it's off.
Last thing is, particularly since your snake is so small, you might want to add some more fake foliage or paper towel rolls or other permeable visual barriers to the open area. If she's eating for you she's probably fine, but if she seems skittish or reluctant to feed, cluttering up the empty space might help.
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