I agree that old gods can be in two places at once, or rather, they tend to be so massive that we may observe them in multiple places at once. We know that C'Thun and Yogg as we saw them were not the entire body, we only saw a fragment. These faceless ones have clearly defined heads and arms that we can see, and seem to share a model that we've seen on occasion and we see frequently in Cataclysm, and that model has clearly defined legs as well. That's the only reason that I don't believe those two things being excavated by the Twilight Cult are connected. Also, if they're connected, then excavating them kind of seems a futile attempt, no?

Of the little that we do know about old gods, the Master's Glaive thing just doesn't fit. From what I've seen of the quests in that area, it's never said that it's an old god, and hinted at strongly that he was one of their stronger servants. As far as his abilities, this is the first time we've seen a creature like this. We don't really have a set standard of measure for power for him or many other things. Archimonde and Kil'jaeden are both very powerful and have demonstrated some unique and god-like abilites on several occasions, but we know that they're not gods or Titans. And no offense, as I do enjoy Know Your Lore, but there is no lore precedent for Avatars being a signifier of a god. We've seen lesser avatars and images created. It makes sense to assume that given enough power any being could create a rather nasty avatar.

My ultimate theory is that Yogg and C'Thun were both lesser old gods, sort of like Titans that we know are titans, but not part of the Pantheon. From the War of the Ancients, it doesn't make sense to assume that Yogg and C'Thun were included in those old gods planning on usurping Sargeras' Well of Eternity portal to escape. Rather, I think that those old gods, the true head of the snake, if you will, reside on a plane that was also suggested by Bornakk in the latest round of Ask a CDev questions.

Either way, the Twilight Highlands quests will hopefully shed some light on all this. I expect it will reveal that these creatures are some sort of Faceless One generals.