Tom -- I found your post interesting and the premises offered quite likely.
My thoughts on "Celtic creation" fall into the same category. I think we
can't expect to find an ex nihilo creation, in the manner of Genesis.
Instead, what little evidence there is suggests an already existing
"ground of being" in which action is taken and features are created --
lakes bursting, mountains being piled up, plains being cleared. Some
Welsh legends (the white sow) offer origins of species by birth from an
otherworldly source. It would almost seem that different bits were
"created" by different local deities, in varying ways and at different times.