I'm not entirely sure what to think of Adamantine... On the one hand, you've got the commonly-accepted lore of it being some sort of divine "plug" placed by the ancient gods in order to seal away the hordes of hell, but on the other you've got the new version which shows a distinctly less plug-like nature of the stuff. Plus, the "hell coral" theory is simply too delectable to discount without good cause.
I'll have to meditate on this for a while.
Then we get to the prospect of Slade... Personally, I'm quite content with it being an impenetrable material. I don't want to be able to send some dwarf with a blue pick down there to carve up the stuff, I want to have landscapes of a material I have to ruddy well respect.
One possibility is that Slade is the condensed hatred of all of hell's demons. The pure, undiluted loathing they have felt towards the higher worlds has, over the millenia, transmuted into a vast, barren wasteland that not even the demons themselves can escape from (or, at least, that's what we'd like to think).
As for the temples, they may form up around the sword, not the other way around. If we do make the assumption that Adamantine is some sort of divine or supernatural energy-turned-matter, then it could be that it was fashioned or "focused" into weapons, which were used to beat back and imprison the demons in the ages before time.
Some of these relics could have been left behind as the forces that wielded them vanished from the world. The demons would be left with nothing to blame but the leavings of their oppressors, and this focused, concentrated bile would form a vast and chaotic hall around the weapons.
This doesn't really explain the undead though... Possibly hate-spawned shadows of those mortals who fought in the ancient wars? But then we're still left with the passageway left behind by the weapon... Hmm.
Well, it's all rather interesting, but now I'm afraid I need to go smash some things into the lava pit. Ta ta.