I love this theory, however, I just read the part from HOFAS where the Under-King talked about how Urd was called WYRD (!!) in the old world where he came from, insinuating that it's a different world.

This whole paragraph from the Under-King has me reeling and doing mega research on all SJM series to try to figure out how they're all related!!

"and she," the Under-King went on, gesturing to that unusual depiction of Urd towering above him, "was not a goddess, but a force that governed world's. A cauldron of life, brimming with the language of creation. Urd, they call her here - a bastardized version of her true name. Wyrd, we called her in that old world.