Have Mr. Dershowitz explain the apparent contradiction. Perhaps there’s some penumbra involved, or perhaps you only quoted part of what he said. To me, a majority of an unspecified number does not have to be a certain number. If fewer than 538 electors are appointed, for whatever reason, then a number smaller than 270 can be a majority. Sorry if you don’t like it, but the 12th Amendment says what it says. Majority, not 270 or any other specific number.
What you quoted of Dershowitz’s statement might still not be in conflict with the 12th Amendment, if the total number of electors appointed were 534 or 536 and if they voted in equal numbers for Trump and Biden. Then it would go to the House, just as the 12th Amendment calls for.
Ya, and if things were different then they wouldn't be the same. The electors were appointed before the election. There are 538 of them. The magic number is 270. If nobody gets 270, it goes to the house, whether there is a majority or not. If some state decides to not send their electors, it DOES NOT change the 270 threshold.