Just a theory, but since the great Jerome Bixby shares the teleplay credit for this episode with D.C. Fontana, wouldn't it be possible that she's somewhat responsible for the shift in tone at the end of the episode?
Not to belittle Fontana's work (I've always been a huge admirer of her's), but the tone definitely doesn't befit Bixby's other efforts for the series, which were always more earnest in tone. The light tone actually seems more in line with Fontana's work.
So my theory is that there was a moment during the writing of this episode, where Fontana rewrote Bixby's script or "took over" from where he left. If true, I wonder what the original ending looked like. Is there someone here who knows more about what happened during the writing?
Anyways, "By Any Other Name" is one of my favorite episodes not only from the second season but the series as a whole. The Kelvans are good villains (especially Warren Stevens as Rojan), the pacing of the episode and the blocking of the scenes is wonderful. This is just one of these episodes I never get tired of watching.
Seeing as no-one ever really followed up on this, I guess I'm the only one who cares, but I just found a bit on the MA
page for this episode, where it is said, that Bixby once told an interviewer, that he initially wrote the episode in a much more serious vain, but that Roddenberry stepped in and ordered a re-write by Fontana, who made it a somewhat "lightweight" story.
So the theory I had posted earlier is actually true! That's very interesting and makes me want to read Bixby's original outline/teleplay even more. It's a pity that no-one seems so have anything like that. I wonder if Bixby's son, who himself is a film producer, would know something about that.
EDIT: One more thing: The MA article refers to a book called "The Star Trek Interview Book" by Allan Asherman. Does anyone have this book lying around? What else does Bixby say about "By Any Other Name"? Does he also mention "Requiem for Methuselah"? Anyone care to share a scan?