I rather think it was only the Bosmer/Khajiit who had trouble keeping their shapes. None of the other races have this myth, and none of them made pacts with Azurah or Y'ffre or anyone else to learn how to overcome this Ooze state.

It wasn't just the Bosmer and the Khajiit. The land itself was also said to be shapeless, and there are other references of untime appearing to be formless to the outside observer. Then there's also the example of characters shifting in Shor Son of Shor. Finally, MK implied that any Mer is capable of participating in the Wild Hunt, not just Bosmer.

Once, there was nothing but formlessness. The land held no shape, the trees did not harden into timber and bark, and the Elves themselves shifted from form to form. This formlessness was called the Ooze.

We watched our borders and saw them shift like snakes, and saw you run around in it like the spirits of old, devoid of math, without your if-thens, succumbing to the Ever Now like slaves of the slim folly, stasis.

Stuhn and Tsun were shifting and it was still uncouth to prevent this kind of neighboring.

During the Oblivion Crisis, the Bosmer were going to call a Wild Hunt to end all Wild Hunts, with every single mer in Valenwood going full monster.

According to the Wood Elves, after the creation of the mortal plane everything was in chaos. The first mortals were turning into plants and animals and back again. Then Y'ffre transformed himself into the first of the Ehlnofey, or 'Earth Bones'. After these laws of nature were established, mortals had a semblance of safety in the new world, because they could finally understand it.

The reason Bosmer are the ones that most prominently mention this in their myths is because their ability to shape their surrounding plantlife has direct ties to the pact they made with Yffre. The other races don't talk about it as much because after Yffre became the earth bones, they became ordered and that was that. They had no need to make a pact with him to shape their surroundings.

Referring to Adamantia as the Zero Stone would just be a case of poetic license, of metonymy. Like saying "He died by my hand," when you actually used a sword instead of your bare hands. Or like saying that a car moves; it does, but only when there's gasoline involved. But you don't feel the need to specify that the car and the gasoline move, or that the gasoline moves the car, every time you talk about the car's movement.

Yes, but you wouldn't say something like "a car, also known as the engine" or "a human being, also known as the brain" or something like that. It very clearly refers to Ada-Mantia also being known as the Zero Stone.

Which is directly in conflict with the reading of Adamantia and Zero Stone as the same thing.

I don't necessarily see it that way. Ada-Mantia, Ur-Tower, and Zero Stone could simply be different functions of the same structure. Like how the heart, lungs, and brain are all different functioning parts of the same human body.