Sounds like you might have confusion about how radiant heating works and how it indirectly controls ambient temperature.
Radiant heat panels, ceramic heat emitters, and even light bulbs all work the same way. They radiate outward in a mostly straight direction and only heat what they come into contact with. That heated area then radiates back up and indirectly causes air in a confined space to warm up as you slowly get a heat exchange in the inside air as it circulates (hot air rises).
The heat build up you are seeing is probably normal and you are over thinking this.
Heat will not disperse from a radiant heat source, that is only accomplished through active means such as convection heat (the pushing of air via external means over a radiant heat source). If you are concerned with the floor temperature under the CHE you have two options, lower the wattage going into the CHE to lower the heat output or remove dark objects under the CHE until the area cools. Be warned however you may negatively impact the ambient temp you have.
The drawback is that since you are using the CHE to indirectly impact ambient temps, you need to have it heat an area a certain temp to then radiate heat back up and impact the air surrounding that area.
Ignore the tank walls, those are irrelevant. The only areas you should concern yourself with is the inside floor temp under the hot hide where the UTH is (never measure the substrate above the UTH, always measure the inside floor temp), the ambient temp between the hot and cold hide (you need a temp probe to do this, IR temp guns won't do jack to measure ambient temps, they don't work that way) and the substrate temp surrounding or inside the cool hide.
No other part of the tank matters as far as taking temps is concerned.
Your CHE should be some placed between the hot and cold hide. Wherever it can heat indirectly the ambient temps the best. The hot spot under the CHE is also mostly irrelevant so long as it does not reach a temp that can cause skin and scale issues. 90 degrees under the CHE is pretty good and not sure what you are worrying about.
In short, you are way overthinking things. I think you are fine so long as your cool hide is at least 10 degrees cooler than the warm hide as a guideline and never below 75 degrees for an extended period of time (as in days or weeks).
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