I agree with your theory on Goldberry, Astaldo-at least her cultural background-but also she could have been a Druid or a Celtic spirit or a nymph or something along those lines(maybe an Atlantean because she springs from water?).
As for Tom Bombadil, these are my feelings:
If we look at the stories alone as solitary tales, unrelated in any way, then a lot of the places or characters do not seem to fit in with each particular story; i.e. Beorn, Bombadil, the Ents, or even Old Man Willow. However, we know that these tales were related in that they convey a larger picture: the whole of Middle Earth, its history, its races, its gods, its enemies, etc. The fact that they are not all cohesive or complete is very logical-no history is ever all-encompassing. Certain facts and events are non-intentionally omitted because to include EVERYTHING would be impossible, because by the time the recorder finished recounting a certain era, said era would have passed. Does that make any sense?
If we look at Bombadil as a detail, rather than an anomolie, then he becomes essential to the recounting of the history of Middle Earth, regardless of his relevance to Lord of the Rings. He is as important as any of the other details-colors, scents, armor, architecture, poems/lays/songs, the different tribes of men, locations, etc. and so forth. Without any of these details, you would be left with a hollow story, and one not half as good as it is. It is the details of a story or history that make it both interesting and accurate. A detail examined by itself has no meaning, but when looked at in a broader scope in relation to the Big Picture, it becomes not only important, but inherent to the story's power to hold our interest.
What I am trying to say (in a very rambling manner) is that the recounting of the story of the War of the Ring, and all that precedes it or follows it, and Middle Earth itself, would not be complete without Tom as he lends richness of detail to FOTR. Also, as I believe was mentioned before, he does serve to give both the hobbits and the reader a rest-all of the events prior to and after they meet Tom are quite anxiety-ridden; Tolkien knew this and probably wanted to ease our reading a bit.
As for Tom's origin, I don't have a clue, but the most logical theory that I've heard so far is (Grondy's?) the idea that TB might be a corporeal incarnation of Eru. That, to me, makes the most sense.
A very good post on good old Tom. Congratulations Laurelindhe. 100 pieces of Mithril will be coming your way when we are able to do so.