It's supposed to be (from what I understand) an infinite amount of timelines happening at the same time, all of them being true, with all spirits being every possible thing they can be in each one, which is why it's said to be formless. It was literally a period when anything could go. I think this was the whole point of the words "the impossipoint of the Convention was the first." The nothing (or in this case, the anything) that turned into something (linear time) was the zero stone.

Mmm, I wouldn't agree to this picture of the Dawn Era. I certainly don't think any spirit could be anything, for example; the Eight were incorporated for reasons, specifically because they had power over certain Tones and could combine to make a certain kind of Mundus. And I used to believe that Dragon Breaks necessarily were infinite in scope, but I really don't anymore, and /u/Mdnthrvst was the one to strip me of that notion way back when. I'd show you but it was really such a long time ago that I don't know if I could dig it up again.

The point is, though, that non-linear time doesn't mean, to me, that there are no rules and that anything is possible and therefore everything happens. It just means that causality is given a bit more wiggle room in the axes of Time. And I see Convention as the ultimate pinning down of that causality, as Auriel saying, "No, look, if you come here, there's going to be ONE way this plays out, and none other. And it's gonna be that way permanently." The spike of Adamantia was, in other words, the first step away from the relative (but not total!) chaos of the Dawn, and it was a very, very big step. So it doesn't make sense to me, on at least two different levels, to say that Convention is somehow equally subject to the non-linear nature of the Dawn, and that this non-linear nature means all things are real.

So here is a new idea: what if what the Direnni are touching in the depths of Ada-Mantia is the original metallic casing of the true Ada-Mantia?

That's a rather odd reading of Once! It seems more likely to me that the Zero Stone is in the Foundation Vault (or perhaps that the Foundation Vault is whatever is behind the Aperture, and the Zero Stone is outside it), and the metallic wall was just description for the actual wall the Aperture is built into.

And, come to think of it, I don't think they would even call it the Zero Stone if it weren't an actual, y'know, rock. They'd give it some other name if it were a wall. None of the other Stones are referred to as "Stones" except in a general sense, when describing their relation to their Towers. They're always called what they are: The Heart of Lorkhan, the Perchance Acorn, etc.