Also, having those gem sockets is really nice. It's a small detail and rather sad I know but it's one of the reasons I really enjoyed t10. Having more customisation feels good, even if there is only really 1 right answer.
Firstly, most of those are Tier pieces, not off pieces.
Secondly, I think those items are going to be from Valour Point vendors, not boss drops. They're called all "Valorous" so it suggests they're going to be from Valour Points, which makes sense considering VP vendors won't sell Tier gear anymore, so they'll need to sell more pieces of non-tier gear to compensate.
Thirdly, about the rep thing. I don't think they're placeholders for a new faction at all. I think they've just not been editted properly yet. As far as I'm aware, there's already higher item level Avengers of Hyjal gear in the game files, they just never used it. I think Blizzard then decided to edit the stats and name of the item (only put Valorous in front of it, other than that it's the same name as the one from rep) to save time and not leave useless files lying around.
Edit: I'm not too sure about the Horrific pieces however. I think it's more likely that they'll be from a new rep faction.