Yeah two inches is probably too much. one should be just fine. Make sure you put the probe right on the glass UNDER the substrate. 94* should be the hottest point in the entire tank, not the substrate surface temp. 83* is a good ambient temp.
They say not to use them on plastic because they can heat up to 115* or more unregulated. As long as you are regulating it, your tub and snake will be just fine. I have used a zoomed heat pad with a tub just fine.
These are normal questions and they get asked frequently.