Yes, now you're starting to see the problem. The Dark One, in fact, cannot win. There is a possibility that the Dark One remains imprisoned in some world, so therefore he remains imprisoned in all worlds. This does seem, at first glance, to be more of a paradox than it really is. The Dark One is actually imprisoned outside the Pattern, which all worlds that are or that could be are a part of, so there it is actually the same prison that is referenced in each world.

As you say, it does seem like there should be a possibility of the Dark One winning in some worlds, in which case he would win in them all. Ordinarily I would agree; there really shouldn't be any exceptions to infinite possibility. Illogically, that isn't the case though. Look at the Dark One's stated intentions: he wants to break time and undo creation. Because one Pattern contains all of the various infinite worlds, unraveling it in one would cause similar annihilation in all of the others. Clearly, that hasn't happened.

There are two possibilities I can see for this. The first is that the Dark One simply cannot ever achieve his victory. The other, which is my personal preference, is that when the Dark One does win, he still loses.

Let's take a look at a hypothetical victory for the shadow. The Dark One breaks free of his prison, breaks the wheel of time, and unravels the Pattern. Creation is undone as if it never was, and chaos reigns. Almost. We would still be left with two opposing forces, the Dark One and the Creator. And we have never been given any indication that the Dark One can actually challenge the Creator. So, the Dark One wins and establishes a multiverse of nothingness. The Creator would most likely respond by imprisoning the Dark One to prevent his interference. Then he would live up to his name and create something. So the cycle repeats.

So there you have it. The Dark One can't win, and if he ever does win he still loses. It's all actually quite simple. I would say that it should be intuitively obvious to any brilliant but not entirely sane philosopher, such as Elan Morin Tedronai, Herid Fel, or myself.

I'm not quite sure what you meant by over-thinking though...

I'm sure people in those other realities think that theirs is the only one that matters too.