I refuse to believe Ruin caused this prophecy. I've explained before that Ruin, as a force of chaos and destruction, does not have a "creative" quality to actually create such prophecies. Ruin didn't create the Terris Prophecies; if he had, then there would have been no need to manipulate them to suit his needs, because the prophecies would be made to suit his needs to begin with.
Besides, I am of the opinion Ruin had no power during the Lord Ruler's reign. It was the Lord Ruler doing something that prevented Ruin from spawning the mists which killed and manipulate the prophecies. The Lord Ruler halted Ruin from acting... if Ruin could act at all, it would be infinitesimally lesser in quantity than seen in MB2. The Steel Inquisitors were not controlled by Ruin during the Final Empire, so it is colossally unlikely Ruin had any power at all.
As for your case #2, I doubt it's likely that someone really "foretold" the Lord Ruler's downfall. It could just be some random Allomancer hobo who discovered malatium and not realizing that there weren't 16 metals, thought that this special, new metal which was separate from the main ten was important somehow. Combine that discovery with the longing to end the Lord Ruler's, it's not difficult to think that somebody just discovered malatium on accident and created this theory on a hope. This person wouldn't randomly tell people about this "prophecy", so that would mean Sazed would have never heard of it.
As a corollary, if the Allomancer Hobo had a distrust of people which prevented him from telling this theory to, say, Keepers, then the Allomancer Hobo must have trusted Kelsier (or Gemmel, Kelsier's mentor) to give it to him. And I believe the prophecy is rather vague because Kelsier didn't really know how to use it. If someone discovered malatium on accident as described above, I doubt he would know the deep intricacies of the metal, much less how the metal would defeat the Lord Ruler. It's a prophecy built on hope, not strategy.
(Note, I call him the Allomancer Hobo because he would need to be an Allomancer to ensure the metal was of Allomantic quality, and Hobo because it could be any random person who could have discovered it.)
Of course, it could be the kandra, but somehow that appears far less likely than this scenario.