The mists, prior to the Ascension, existed solely in the form of the Deepness
How do we know this? In one of Alendi's bumps he states
We are close now. Oddly, this high in the mountains, we seem to finally be free from the oppressive touch of the Deepness. It has been quite a while since I knew what that was like.
The lake that Fedik discovered is below us now—I can see it from the ledge. It looks even more eerie from up here, with its glassy—almost metallic—sheen. I almost wish I had let him take a sample of its waters.
Perhaps his interest was what angered the mist creature that follows us. Perhaps . . . that was why it decided to attack him, stabbing him with its invisible knife.
He mentions both the Deepness and the mist creature but does not connect them. If they were one in the same, he would have said that it was the deepness that attacked Fedik.
If you read The Mists topic, I say that if the Mists are foundational to the magic systems, and we find no evidence of mist-based magics (Allomancy and Hemalurgy) pre-Ascension.
Alendi had the Piercings of the Hero, possibly being Hemalurgy related. I agree however, we find no evidence that these existed pre-ascension, however lack of metion is not proof that they did not exist.
The Lord Ruler could have done something to the mists themselves to allow Allomancy and Hemalurgy to occur.
Totally agree.