Armed with notes from years past, I did not even dribble candy into a glass of water as the temperature rose. I couldn't find my old-fashioned glass thermometer, so used the fancy William Sonoma flat metal one. Then the fateful comment was made: "The one thing that hasn't gone wrong before was to go too high on the temperature".
So, nothing to do but start over, after a good 20-30 minute hunt for the thermometer. This type, but I don't think this brand, just whatever was at the grocery. Also, I used a spoon and glass of water to check the thread, soft ball, and hard ball stages. I also used my fancy electronic thermometer, the only one I've found that works with the induction cooktop.
The glass thermometer rose steadily. The electronic was more volatile, reacting quickly as the probe was moved around in the pot. As the glass one approached 260, the electronic would read as high as 270! I finally couldn't take it any more, and pulled it off the heat when the glass one read 255. And, that was too early.
The candy was too sticky to hold for long, and was really soft. I laid it back down on the silicone mat, and pulled/folded it on the mat. Then, in a surreal few minutes, it turned foggy, smelled wonderful, then turned into crystal and broke in pieces. Success! Although not beautiful, it was all gone in a moment, as many hands reached in.
Bonus: By the next day (today), the original batch crystallized into a slightly dryer, more crumbly version of the proper texture. I just checked, and it is softening, and is almost just right! The kids say that our true belief that the batch had failed is why it turned good. Hahaha, and my brother's family took some with them; should be witnessing the magic also, on their journey home. A good Thanksgiving was had by all.