Yep, that's a good small conclusion.

I'm not familiar with that brand of UTH, but it sounds like your substrate is too thick. If there's a 15-degree difference between the UTH and the top of the substrate, there's something wrong.

I'm using BN Link thermostats (they're cheap) and it has a 3 degree range, so I set the UTH to 93 and it kicks on when the temp drops to 90.

I'm using an Aiicioo UTH, and it's worked just fine, I don't have problems with it.

As far as the doming, I'm not sure what you mean. You said the probe is in the center, right? Between the mat and the glass?

Doing this will create a lump in the center of the mat, where the probe is, but the entire mat should have adhesive on it, so it should still stick to the glass just fine. You don't want anything else holding the mat to the glass. If you don't allow air flow, the uth may overheat even with a thermostat regulating it (I learned that the hard way once, when a big uth overheated and cracked the glass in my 100 gallon retic tank. Paying to replace that wasn't fun).

But yeah, there will be a lump, but I don't know what you mean by doming. If it's not staying against the glass, maybe your mat is too small or the adhesive is crappy. It may be worth getting a different one.

Yeah, the CHE thermostat probe needs to be inside the tank. I keep it about 2 inches above the substrate, in the back corner (hidden from view by the warm hide), directly under the CHE. My girl leaves the cord alone, so I just use the built-in cord hole in the top of my tank and slide the lever over to keep the cord taut and straight. If your snakes are more active, you may need to actually adhere the cord to the glass. And if the probe is right against the corner, it's very unnoticeable so it doesn't mess with aesthetics. Especially if you've got the hide in front of it.

Because yeah, I totally understand what you mean by aesthetics. I'm a sucker for it, as well. That's why I feel that the finicky-ness of a glass container is worth the extra work. It just looks so nice. I've got my girl on her own table in my kitchen, and it looks great. And it's so freaking cute when I'm cooking and I glance over and see her face at the entrance of her hide, just watching me.

I can absolutely appreciate tubs for their ease and practicality, but glass just looks amazing. Not even pvc can beat the look of glass, in my opinion.