Like you said, I hesitate to think any of the Winchesters as a Supernatural creature (yes I see God as one of those too). But I'm okay with them being the metaphors of these beings.

Although one thing about Michael kinda bugged me since 5.01. Why have we not seen Michael yet? Zachariah said Michael Sword was lost. So does that mean once the angels had it/him? In other words, was Dean among the angels at one time? If that's the case, that would make Dean more like Anna/Anna than Jimmy/Castiel, wouldn't it (assuming Michael Sword and Michael one and the same)?