Sorry to hear this, good luck with survivors, there may be a few more you haven't noticed yet as well.
btw this; "the other week, caught the hydor roller switch and knocked it right up."
sounds like bad design. Once your temp is set right, could you put a spot of silicone (or other easily removeable sticky stuff) on the switch to keep it in the right position?
In a big tank, or where the livestock is valuable or much-loved, it's worth thinking about using two heaters. You use the same total wattage, but if one sticks 'on' the tank will only overheat very slowly (if at all) as only half the heating is stuck. And if one fails, the other will slow down the temperature drop, giving you more time to notice and fix the problem.
That protection device you mention sounds like a good idea too, but I've never tried one.