I'm going to give my two cents on these theories here:
The one that the Gang died and the Great Valley was just a metaphor for heaven is an interesting one, but honestly, I think it's baloney. That one would hold a lot more water for me if none of the sequels had happened. I hear some fans don't consider the sequels canon, so maybe it holds more water for them, but I do consider them canon, so I think the Gang is alive and well in the Great Valley.
Redclaw is the original Sharptooth?! Oh man, that one just cracks me up!! I think Redclaw is an utter joke compared to Sharptooth, so I think it's totally preposterous that Redclaw is Sharptooth.
As for the rainbowfaces, I think they're definitely aliens. It's pretty obvious that they're some kind of supernatural beings, and they strike me as being aliens. I've heard some people say they think the rainbowfaces are angels, but they seem more like aliens to me.