Roman said:
Thanks everybody! So it seems that the common origin of these myths and legends ensures that although they are not exactly the same, they are similar enough to be grouped together as one mythology/legend system.
As much as any common cultural system can be after generations and generations of spread over a large distance, evolving influences from neighbors, and gradually diverging languages. For what it's worth, we can easily create root words for proto-Germanic forms of many of the common gods of Germanic mythology --
*Thun-raz for Thorr/Thor/Donar, etc. I don't know how useful that is, though.
I mean, it wouldn't be hard to go even further with it, and try to reconstruct a common Indo-European mythology, noting the very obvious similarities between various Indo-European thunder gods like Vedic Indra, Slavic Perkunas, Baltic Perun, Germanic Thor, etc. In fact, there are researchers, generally following in the Georges Dumezil school of thought, who publish material doing exactly that.
So, there's no cut and dried answer, and at what point you decide, "this is too different; this is a new tradition" is pretty arbitrary. Personally, my position on it is that "Germanic" mythology is common across the entire pagan Germanic tradition, with obvious regional differences. Not only do we have the problem noted above, in which we don't know how much of the recorded Norse mythology is actually what any Norse believed vs. Sturleson's own interpretation and propoganda spin on the mythology, there are other vagaries that are interesting.
For instance, and I really wish I could remember where I read this, Odin didn't really used to be that important of a god, and he certainly wasn't the "All-father" until his cult, which spread from Germany, actually, pushed him forward. Before that, Thor was the king of the gods in the Germanic tradition. Another intriguing proposition I've heard is that the Vanir are not a native Germanic tradition, and represent increasing contact with Celtic mythology; in other words, the Vanir are the Celtic pantheon, and the Vanir/Aesir war represent relations between Germanic and Celtic peoples! I'm not sure how much I buy that idea, but it is intriguing.
The thing about oral tradition religion that has been "dead" for centuries is that finding the "definitive" version of it is impossible, and not even desirable, in my opinion ayway.