I think all you really need to do is buy a thermostat and hook it up, as this will keep the heat mat at a safer temperature. There's nothing wrong with a heat pad that reaches over 90 degress, so I doubt you'll be able to return it as faulty.
You could maybe move the tank to that, say only one-third of it is above the heat mat. That would give the little guy more opportunity to cool down while you get the stat set up. If you go for a tank, the recommendation is that only one-third of the floor area should be heated anyway.
Measuring the temps at ground level is exactly the right thing to do, as that's where your heat source is. You'd be surprised how many people try measuring air temperature a few inches above where the heat source and snakes actually live! When you get it, the stat sensor should also go on the tank floor, immediately above the mat.
Good on you for stepping in and saving the little guy - keep us posted on progress.