Sorry, I'm still confused...lol
So if you put the tstat probe into the substrate, why wouldn't it work the same? Doesn't the probe take the temp of whatever it is touching and adjust the tstat accordingly? Isn't that how it works; basically the tstat probe IS a thermometer.
Say I put my probe into the substrate and set my tstat to 90 degrees. So then wouldn't the probe measure the temp of the substrate until it is 90 and then "tell" the tstat to turn the heat mat off? So why does it matter whether you put the probe on the mat or in the substrate?
I would then use my thermometer as a second check.

also Bumble shed and he's beautiful now