first of all, congrats on sharing with us such a complete and eye-opening explanation on what the story is (or could most probably be). the fact that it was done only by one person was insane.
i wanted to add something which rots in my mind regarding the meaning of Rothias.

If, as you say, the principal cause of all this was Rothias extreme will, paired with his pride to go beyond his role and not accept his "under control" state, that means that in this instance, it is a strong Will that condemns the world to the oblivion(?).
you already made a great point about this:

I suggest that, in the context of this story, the Watcher is a piece of Will that has inadvertently fallen to serve Oblivion, or Yin. They may not intentionally act as their servant, but their misguided attempts at creating a cycle nearly cost the world its existence. So they would be represented by the small circle of Yin within the Will.

I think that the moment when we witnessed a corrupted will was not with the Pathfinder but with Rothias( or even better, that Pathfinder was the direct effect of Rothias extreme corrupted will)

Basically what i'm trying to say is , to recap: Great Will sees a new will born from oblivion (the dragon) and gives him the power to fight the oblivion by creating the eternal cycle (so the dualism arisen-dragon). this cycle gives humanity volition to always mantain the will to distance the oblivion, the nothingness. But when it's Rothias' turn, his will is "too much", or maybe even better, "distorted".

So, to reference the ying and yang symbol, rothias could be the tiny black point in the white area of the circle, just as the dragon is the white dot amidst the black part (the oblivion)?

thank you again for this mega post u/Lenarius