Love threads like this; a joy to read and by gosh even makes me think!
Tolkien may have patterned Eru after his knowledge and understanding of the Catholic God, yet to say that Eru is the God of Adam, Noah and Moses is a bit of a reach. Even when imagining that the Sil and LotR are history, I have a hard time reconciling Eru with the God of the Old Testament. As an example. the Ainur and Maiar aren't angels but more like relatives - cousins. Now think about it. Unless you misinterpret
Genesis 6:1-4, angels do not have offspring with humans. Melian and Thingol's daughter marries Beren.
And there are some Christians that believe in a preAdamic race of humans that were before the Fall. Are the peoples of ME these people? Or all they all post-Adam? I have a tough time reconciling either.
Anyway, not to offend, but my point is that if you label Eru as Christian, then you have to buy the whole meal, not just sample from the buffet and call it dinner.