I know what happens in the Alt. Ending, I saw it myself, but I was still in doubt about that physical part, what happened exactly. I suppose one could certainly see it as a sex scene, - i must admit it certainly sounds that way - and you'd have to see the flames n stuff as something symbolic.
But it could also be that during this strange merge-intercourse-thing the prince (and kaileena too, maybe) had a vision of what awaits him in Babylon, because the history changed big-time.
Don't forget, no sands, no hourglass, no foul-hearted vizier (well, maybe he tried to take over power in some other way) no dagger, no amulet, no love affair with Farah! It would be as if PoP1 and 2 never happened, and the prince would still be the young lad we saw at the start of PoP1: with a craving for honor and glory. So India might still be conquered (that would possibly explain Farah hanging on a piece of wood in the alt. ending).
Now the "mature" (that is, depressed) prince who went through the horrors of Sands of Time and WW, comes back, with Kaileena, his newly found love, thinkin he's got it made (especially after getting oral sex from the Empress, see alt. ending) coming back and seeing, well, that history is really screwed up.
So screwed up that I forgot my point and don;t even get my own theory anymore. 😕 😮 🤣
Oh well, Ubisoft better not let us down with PoP 3!!!!!!!