It means Yeul is the physical form of Chaos.
I don't know where you're getting Yeul = Mwynn from... @_@
Because Mwynn pretty much could control when she melded with the chaos. Of course, that is if you take how she waited for Etro, bestowed upon Etro her powers and a focus, and then all of a sudden vanished into the chaos.
Of course, the original myth said she was melding into it when Etro found her, I believe. Hmmm... I just still think Mwynn was one with the Chaos. Either she was the original embodiment or she was a direct child of it, she seemed to be able to control it.
That's me though. My thinking. Until the Ultimania(s) say otherwise...
I tried looking for fal'Cie Pandemonium in Yusnaan but failed.
He's the man-made one in Yusnaan that supplies food to the world. He was briefly mentioned in the game and is apparently the last active fal'Cie.
I don't think there are any more man made fal'Cie. Adam was to be the first, and last, then never got to be at all. And even in that, Adam was really a part of Eden! (My reason for disliking Hope.)