+2
I think "she can take a wight's face" is the perfect surprise, in that it's something most of us won't have considered, but which makes perfect sense within the rules of that power. I certainly can't think of a reason she wouldn't be able to. Anything too obvious and it's not cool or surprising, but anything not obvious enough and it feels like a cheat. Obviously, this'll vary a little from person to person, but I think that idea satisfies the same criteria as a good plot twist: you won't think of it, but in seems obvious in retrospect.
I honestly don't entirely follow what you mean by the hypothetical where she runs up behind them. I thought he looked well-nigh(t) surrounded, though maybe there was an opening (if so, why?) in the pile of the dead. But to be clear, I'm already granting that she can move quickly and quietly.
Anyway, I dunno that this rises to the level of plot hole, or whatever. I just think any good surprise has to be more than just technically possible, or impossible to disproven. Just seems like a low bar to clear for what's supposed to be one of the coolest and most exciting moments in the show's history. Feels like they left some pretty cool options on the table, but that's just my opinion, for sure.