To make the setup work, aye

Well, you could consider the solution to the riddle a pun though that's quite a stretch. Though there is at least one modern rendition (in German) that directly uses a more pun-ny solution. It does't quite translate to English but I'll try. Basically, Oedipus thinks and thinks until he starts to complain "Ach Mensch..." which is roughly equal to English "Oh boy..." but "Mensch" is literally the German for "human" so it's the right solution and the sphinx has to accept it. If you understand German, I highly recommend checking out this version. It's "König Ödipus" by Bodo Wartke. He plays all roles himself on a mostly empty stage with only a couple of props and it's absolutely hilarious. The sphinx is a lion hand puppet.

Edit: for anyone interested, here's a youtube clip of the scene: https://www.youtube.com/watch?v=DogC57ZJuY8 (German with German subtitles)

I can confirm this to be correct.

Way less gross. Human and centaur are both intelligent, can communicate, and give consent, so it would be fine. With a horse (which has none of these things) the centaur would be committing bestiality.

Apply the Harkness test (self aware intelligence, physical maturity, and ability to communicate consent). Both humans and centaurs pass assuming the individual is a willing adult, horses fail twice due to being nonsapient and uncommunicative.

From the human perpective, probably the horse is a little bit less gross. From the centaur perpective, probably the human is less gross, but I’m only half centaur, so can’t speak for all of them.

It depends how it flops.

Does it flop at the first set of legs, or the second?