I have no idea why you're underrating C'Thun Priest so much.
Obviously the vanilla C'Thun minions aren't as strong as other by themselves, but they still give the deck early drops to contest the board that do useful stuff later. The specific support the class got is absolutely fantastic, too. A 4 mana 3/6 is very hard to deal with, especially when you have hero power and PW: Shield. Combine that with the fact that it can make your C'Thun pretty damn big if it sticks around, which it often will, and it has super high value. Darkmender heals a 3rd of your starting HP after very little buffing AND has a massive body. Priest also has tons of heals to survive until late, as well as reliable draw options (Cleric and Acolyte synergy) to dig for C'Thun. You say Priest is bad at playing a midrange style with a finisher, but that has two problems: First, you're disregarding the possibility that the deck can stall long enough with heals, all the new taunts, and vanillas to survive to late game and control. Second, the reason Priest has never been good playing to a big finisher is because the class has never HAD one. C'thun is pretty easy to get big enough to either take out a good chunk of your opponent's board, or straight kill them. And no, the fact that they lost on stream with C'Thun priest is not relevant. It was one game, and there's exactly zero possibility they were playing with an optimized build.
Darkshire Alchemist is WAY better than Darkscale Healer. Alchemist can be a stronger Healbot on an empty board, or keep a big minion alive if you have one. Healer does jack shit on an empty board, and is only useful in the unlikely situation you have tons of damaged minions and 5 mana to use.
I agree that Shaping is pretty good and Shifting Shade is OK. SW: Horror is probably bad, but has a bit more potential than you give it credit for.
Anyway, that's just my two cents (maybe even three, this post is longer than I expected). I could be totally wrong, but I think a lot of people are underestimating the stuff we got here.