: Who's holding the ring on a chain?
Definitely Sean Bean aka Boromir. I'm not sure why he is holding the ring, but I'm guessing/hoping the other end of the chain is attached to Frodo somehow. I think I agree that this shot is prior to the attempt to cross the mountains. Amon Hen doesn't make sense, since there are mountains behind him.

: Who's that very last guy we see who seems to be throwing something?

Aragorn through a flaming torch at the Nazgul at Weathertop. Probably will be morphed into the shot with the hobbits somehow. It's hard to see his face clearly, even going frame by frame, but it's the only thing that makes sense from what we know.

: Any ideas on the really big scene with all the orcs? Helm's Deep?

I think so. Torches, pikes, lots of Orcs. Sounds like Helm's deep. Plus, we know that they have been filming there already (at least, I think we know that).

As to the seen in the woods, That seems to be the whole Fellowship. It's pretty hard to see, but you can make out a bearded character that must be Gimli. I'm not sure what you meant by the thing overhead, though. I don't see it.

: And what's with the chick on the leg stilts??

I like that explanation - used for scaling the hobbits. I was thinking Ents or Trolls, but the leg movements aren't right for Ents.

My 2 cents