Could Demandred's absence be explained by him ruling an entire Mirror World of the Wheel, and readying himself to bring the entire world's armed forces and channeling population over to invade Randland?

Since time flows differently in these Mirror Worlds, he could have been securing his rule there for decades, while only a year has passed in the real world.

In this way he could have amassed an army of immense proportions, including thousands of channelers, to bring across and overwhelm the forces of the Light.

And he would have done all of this without any trace being seen in the real world.