From my reading of that, I'm guessing that the forces of Chaos want to destroy the Material Plane, by allowing the elemental planes to slam into each other again. Do the forces of Law only want to preserve the Material Plane? Or do they too want to destroy it, but by instead separating the elemental planes properly once again?

As an idea regarding the Awakening: While those who recognise the threat to the world that these otherworldly forces represent are usually laughed off or derided as mad, what happens when someone who has the ear of royalty discovers hints of the truth in the ruins of the past? When a small contingent of powerful mages set to work to discover something, rarely does it remain hidden for long. And once the truth gets out, paranoia would spread, as ordinary folk begin to distrust one another, suspecting everyone around them as wanting to doom the world. On a larger scale, this distrust could be used to fuel the fires of old resentments and reopen old wounds, leading to wars as people lash out at a threat beyond their comprehension.

As to the Collapse: Perhaps magic in the Material Plane is tied to the forces of Law and Chaos. Chaos gives magic its strength and potency, while Law gives it form and direction. Without Law, magic becomes extremely powerful, but wild and unpredictable, potentially with catastrophic consequences. Without Chaos, magic drains from the world, perhaps causing creatures that feed on magic to become more desperate, and they take to the surface world and rend it asunder in a desperate need to feed on the magic inherent to living creatures. As to why these factors could cause the extra-planar forces to temporarily abandon the Material Plane, if Chaos is winning perhaps an immense magical feedback loop rips through living creatures, targeting those with the most magic inherent to them (namely, extra-planar beings and powerful mages). If Law is winning, perhaps the forces awakened by the weakening of magic fixate on the extra-planar visitors, hungry for their power more than anyone's.