Eh, I don't know about the yin-yang idea. AFAIK, the Foul One is only interested in consuming and unmaking worlds, not creating new ones. It is the pure essence of nothingness, the antithesis of all that is real, it may not even be sentient as we know it. I think that it is physically (insofar as such an adjective can be applied to it) incapable of making anything.
That's true. Perhaps the yin-yang duality came about because of dwarven spirit. The dwarves even when filtered into an energy-like state managed to have enough drive to survive they they somehow brought the new Aluonra into being. And that Xemorid is simply the left over space where the first Aluonra once existed. Now it is devoid of any natural life and filled only with the Foul One's corruption.
Way back in Failcannon, I had an idea involving a Yggdrasil-like tree. It's fruit were the worlds that DF creates and the tree lives in a weird give-and-take symbiosis with Armok. The Foul One was basically a pestilence in the tree. The tree is rotting slowly and that rot is carried to the fruit, but the fruit of Aluonra rotted on the branch and refused to fall to the ground. The fruit manages to grow despite it being rotten and continues to take precious nutrients from the tree.
Continuing that idea, I guess you could say that dwarven spirit caused a new fruit to grow from the same branch. The problem is that Xemorid (the first Aluonra) is still rotting on the branch and it is touching the new Aluonra sharing its corruption. Xemorid is most like a tumor that is feeding on the nutrients from the tree, but the rotten fruit is taking nutrients that should be going to the other fruit. The people in the worlds think the Foul One has some kind of motivation, but perhaps that is just them trying to understand the motivation of something that can't think.
Ultimately, the problem is the fact that the tree is simply dying and it needs to produce a fertile seed. Any paranoia to the contrary could simply be the unease that the people of the worlds feel because the tree is dying, but can't exactly put there finger on what it is they feel.
The new Aluonra is a possible carrier of a fertile seed, but the fruit is in direct contest with Xemorid the rotten fruit that started the whole thing. The thing is this time, the fruit is rotting from the outside. So the people of Aluonra will feel in direct contest with the corruption. They will live through their lives, create stories, build cities, and dig forts all the while never knowing that their every success and failure brings more life to the seed within the fruit, but any success or headway by the corruption is a sign that the fruit is becoming more rotten. Whereas any failure of the corruption is dent in the Foul One's progress to corrupt.