My apologies for derailing your thread, and I hope I've helped you with your original question.
No apology needed, as, presently, thread isn't derailed. We're still on topic. I totally agree it would be ridiculous for Trump to have to re-do the oath because he didn't finish it by noon.
That said, our opinion on whether or not it would be ridiculous is irrelevant. It's about whether or not there is a constitutional requirement for an incumbent POTUS to complete taking the oath before noon.
As far as I can tell by reading the constitution and the 20th amendment (of course I'm not a supreme court judge or anything and so my interpretation of reading it is, ultimately, irrelevant as well), the letter of the constitution indicates to me that there is such a constitutional requirement. For how can the "term" an incumbent POTUS begin "then" (that is at noon) if he hasn't finished taking the oath by "then"?
I mean, if it has always been the case since the 20th amendment was ratified in 1933 that EVERY incumbent POTUS has finished taking oath before noon, what does it mean that Trump didn't finish taking the oath before noon? Does it mean he isn't technically POTUS?
If so, might an amendment to the 20th amendment be in order? Or, perhaps, a ruling by the supreme court stipulating what the word "then" in the 20th amendment actually refers to?
"Then" could be interpreted to be "subsequently", or it could be interpreted as "at that exact moment". It's ambiguous.