: : My best guess, for what it's worth, is that she was one of the least powerful of the followers of the followers of Ulmo, whose writ ran (Silmarillion) in any water as far as the very springs of streams. Or in classical terms a (fresh)water-nymph.

: So if the River-woman is to be taken as a water-nymph, i.e. a Maia, then Tom Bombadil may have been, as Foster defines him, "a Maia gone native". What do you think?

Can open - worms everywhere!

My view, in a nutshell is:

Maia were Ainu who entered the "Hierarchy of Arda" in a subsidiary role to one of the Valar. The terms Vala and Maia are the equivalent of ranks in an army. As Tom was not (or ever had been) the henchman of a Vala then he could not be a Maia. So what he was was (that's an Anglo-Saxon use of words like had had ;) ) an Ainu who entered Arda, but did not join in the hierarchy of creator/guardians. He enjoyed and observed the created world, indeed revelled in it, but took no responsibility for it (except in very minor ways).

You asked!