The Mirror Worlds are also variations of the Pattern. When he stepped outside the Pattern, he was of all worlds and none. It's not that the reality we've been following is real and the others are all reflections, it's that in the instant of confrontation, Rand represented all worlds. He not only won across all time, he won across all possible variations of the Pattern. The Dark One is free in all worlds, or he is free in none. And he is free in none.
It depends how one defines "reality", but the world in which the story (for one thing whatever they are these worlds are part of the global reality of the universe) is set is the one from which all the variants found in the Mirror Worlds start from. So even if you freed the DO in one of those, nothing would happen. If you freed the DO in reality, this would be reflected instantly in all the reflections.
I'm sure they are other "dimensions"/layers of the Pattern, but IMO they're not real in the sense of having any bearing on the "real worlds", no more than TAR does.
My theory, based on Rand's trip in TGH and other characteristics is that the function of those dimensions where time is somewhat malleable or different if you know how (the Wheel does.. and in one of those worlds Hurin could anticipate what would happen in the real world) is to perform projections/simulations. I think they are the tool the Wheel use to issue foretellings, but above all it goes forward in time a bit to calculate the effects of all relevant variants, which let it pull and push in the right direction to obtain the desired results. That's how choices and freedom can exist despite the Wheel/Pattern: instead of forcing choices on people, the Wheel calculate beforehand the effects of various choices and act accordingly. It's like a giant quantum computer.
We've seen all of this in TGH.. Rand saw that the projection of variants of his defeat over and over and over.. long before that fight took place in the real world. The Wise Ones' Rings also use a similar function, though in their case it's limited to all the choices of a single soul.
Rand and the DO seemingly used the same functions to create their possible realities in AMOL, incidentally. They were "Worlds of If", controlled not by the Wheel but by Rand and Shai'tan, directly.
RJ has basically confirmed the theory of Mirror Worlds/Worlds of Ifs indirectly, that is when asked if the dimension inhabited by the Finns was a "World of If" he answered that no, their realm wasn't a reflection like the Mirror Worlds. There is more than the dimension inhabited by humans, but the Mirror Worlds aren't real in the sense of concrete "stand alone" worlds like the human world, that of the Ogier, or Sindhol. It's been pointed out that the further they strayed from reality the less tangible and logical they became. Even the world visited in TGH was "faded".. with everything the senses can perceive muted.
