If the kumkum turn off on Shabbat, and that the water is still yad soledet bo, can one transfer the content into another pot and place it on the plata.
I heard that Ashkenazim permit reheating water so long that it didn’t cool off completely, how much more so in this case when the water was still boiling. Thank you.
In terms of reheating there is no problem. Even according to Sephardi custom, while the water is still very hot (yad soledes bo) all permit reheating (see Mishnah Berurah 318:22). The Rema is more lenient, and permits reheating even after the item has cooled provided it is still somewhat warm.
However, placing directly on the plata is not permitted according to some authorities, because this “looks like cooking.” According to other authorities, including Rav Moshe Feinstein, this is permitted because one cannot actually cook on a plata, so that it doesn’t “look like cooking.”
Therefore, if the water is still very hot, you can be lenient and place it back on the plata in another pot – though some are stringent and only permit placing it on top of another pot on the plata.