Some more quotes about the idea that it might be Eru...
Finrod, wise among the elves, states that Eru doesnt dwell or take shape in Arda... while later Andreth says
"How could Eru enter into the thing that He has made, and than which He is beyond measure greater? Can the singer enter into his tale or the designer into his picture?"
and Finrod answers
"He is already in it; as well as outside... but indeed the "in-dwelling" and the "out-living" are not in the same mode"
Andreth: "But they speak of Eru Himself entering into Arda, and that is a thing wholly different..."
F.: "For, as it seems to me, even if He in Himself were to enter it, He must still remain also as He is: the Author without..."