Last night, Carrieli, laurelinvanyar, and myself stayed up to ungodly hours theorizing. Though we touched upon many subjects (and laughed until we were all hoarse), what we tentatively determined about how Corypheus came upon the orb was most interesting.
Why would Solas give a blighted magister an orb of such power? Perhaps the answer is, in fact, that he did not directly do so.
Several instances appear to hint to this. For instance, Corypheus never seems to recognize Solas; indeed, if don't play an elf and bring him to the final battle, Corypheus belittles you for bringing along a tattered mage "rattus" (as he likes to call the elves). Should you have Samson instead of Calpernia, in a found note, he will describe how Corypheus once told him the orb came to him for a reason.
Furthermore, we were struck by a particular piece of art seen only once on the war table when beginning What Pride Had Wrought.
Here we see an unknown figure either placing or removing an orb from a remote pedestal in an underground setting. Light and water are pouring in from above. Why is this the art for the Temple of Mythal? We never see a place like this in game. So we theorized, perhaps, what we are seeing is Cadash Thaig, where elves from Arlathan once sheltered after the fall. After all, in Witch Hunt, we encountered the Lights of Arlathan and were forced to fight ancient elven guardians (were these sentinels, like at the Temple of Mythal?) and we all remember the Eluvian. Furthermore, it is a space underground....with surface light pouring in from above.
We believe that upon waking, Solas too weak to utilize his orb, sought out help and found none he could trust. Perhaps then, Solas took the orb to a once safe place: Cadash Thaig. He might have left it there in hopes it would not be discovered as he searched out other options. Unfortunately, around the same time, Corypheus awakened. When our magister takes the body of Larius or Janeka during the events of Legacy, he begins to travel the Deep Roads freely. Cadash Thaig is connected to the Deep Roads, and isn't too far from the Vinmark Mountains anyway. It isn't a stretch to imagine he might have found the orb himself in this way.
As an interesting aside, we also wondered whether Solas sought out Flemeth at this time. We determined that to be likely, but perhaps he did so never knowing she was also Mythal. We know that the Dalish revere the swamp witch as Asha'bellanar and speak tales of her power. We know Solas spent time among the Dalish, and also that he visited Flemeth's hut.