My first thought is that the Pillars of the Earth are the Forgotten Ones/Titans, who are corrupted by the Blight and dwarves/elves are twisted by it into proto-darkspawn. The Creators in their heyday marshal their forces and quarantine the nightmare, but eventually Andruil and Ghilan'nain are infected, and the Pantheon slowly starts to fall into disorder as the gods war amongst themselves and the prison keeps breaking. Mythal is murdered by human upstarts, and things get real bad.
Fen'harel settles affairs.
The 'earth blooming with their passing' definitely points to some sort of Blight related illness. Then again, the Western Approach is still Blighted from the Second Blight, and the Anderfels from most of them. The unsteadiness of Theodosian society doesn't always get across, I think. This is a continent that has faced multiple extinction crises and has lost a lot of livable land, causing them to fight each other even more. It's quite nasty.
Supporting Old Gods as Pillars of the Earth is Solas's outrage at the idea of them being killed. Possibly they can still fulfill their duty even buried. But their Song seems to be untainted, a pure and glorious song, as the Architect wryly notes in The Calling.
EDIT: My sincere apologies for accidentally dropping this off in the 'Smut' portion of the Great Solassian Cycle.
I think you're onto something. Also, what about those pillars that the dwarf in Skyhold talks about the mosaics being made of? Are those the Thousand Pillars? (Sorry, my coffee hasn't kicked in yet and I'm too lazy to look it up.) Anyway, he says that they are made of some sort of mortar, not solid stone.
Also we know that red lyrium turns people to stone, or, more correctly, crystal. What if the other pillars are red lyrium people that have been somehow cured? That would also explain the stone worship of the dwarves.
And I've been thinking about Dirthamen and Falon'Din and the lack of Dirthamen items in his temple, and also the supposedly close relationship they share. I'm wondering if Falon'Din is what Dirthamen became when he was corrupted, and the lore separated him into two people because he was so changed.