to Darkness or falling into it isn't the same as being consumed
Only the latter causes the birth of a Heartless.
Nayru, going by that logic we would also have to wonder why Aqua didn't become a Heartless or Riku in KH 1 whose heart also
fell into darkness.
As for the torment, that was already mentioned in Coded by Data-NaminÃ©.
She warned Mickey and Data-Sora that the combined pain of those connected to him would be too much for Sora to handle and cause his heart to fall to the darkness.
Data-Roxas also alluded to that more than once and this is practically exactly what happened now in DDD.
Like Xigbar said, Sora's heart falling into darkness wasn't caused by any "illusions" or tricks played by him or one of the Xehanort's, they just knew of this fact Data-NaminÃ© mentioned and goaded Sora to tap into the torment of the others unprepared by ensuring he would sleep.
That's true. That must mean he already was revived before his younger self went to the present TWTNW right? Otherwise, YX couldn't be there.
YX could be there because Xigbar was there, he's already half-Xehanort and therefore counts as "existing" because he has a piece of Xehanort.
YX specifically says to Riku that Master Xehanort will be revived just there in the throne, not beforehand somewhere else.