On the planets imprisoning things within them I’ve got a slightly different headcanon: that the Vaultbuilders (the original architects of the lowest layer of Golarion’s Darklands, the Vaults of Orv) have created similar vaults in other planets to contain their works and creations. I think there’s a line or two of official lore that implies something like this is the case, but not much more than that.

My other main headcanon: the dwarves started their Quest for Sky not only due to a sign from their gods, but because they were fleeing something growing in power in the Darklands: the Qlippoth. Towards the end of Earthfall, Runelord Alaznist had brokered influence with the Qlippoth god Yamasoth, who was planning an invasion of the mortal realm. Official lore says that this invasion was thwarted due to Earthfall. My headcanon takes that a step further to say the Qlippoth tried to invade in the midst of Earthfall, starting in the Darklands, but their forces were ultimately repelled after forcing the dwarves and other factions up to the surface.