I think Forgotten Ones is simply the old elven word for darkspawn.

I don't think the Evanuris intentionally created the blight, I think the blight is what happens when you cut off the dwarves from a Titan. According to Dagna, when she had visions of being as tall as a mountain, she felt as though her thoughts were everyone's thoughts, as if her mind was a collection of everyone's minds. We also know that Mythal slew several titans, but also severed dwarves from their Titans. After aeons of mining the dead titans for Lyrium, something happened, and the Evanuris were afraid of what they'd unleashed, so much so that they sealed their Lyrium mines with magic. So what would happen when you remove the mind of a Titan? Say you remove the dwarves from the hivemind, leaving the titan mindless, like a beast driven by emotions. What you get is something that's alive, but unthinking, driven by primal emotions, like anger. And what does Cole say about Red Lyrium? That it's angry.

This goes hand in hand with my thoughts on this subject, whitch I posted on another video. (I personally think the elven "Gods" first brought about the blight in there wars. And it was sealed away in the golden city with the "Gods" by Solas aka the Dred Wolf. Then it was let loose after the Arcons broke into the golden city. This is supported by Coryphaeus's statement that it was already tainted when they got there.) I just had a thought, that what if they used the taint on Mithal and her physical body is still in the golden city. It would go with your thoughts on Solas liking to use poetic justice.

The Blight IS the end all be all in DA, so its conceivable to use it as a weapon However, in the primal thaig, we know that corrupted lyrium is natural - as a form of balance to regular lyrium... maybe it always existed?