My exact reaction as well! AND it took me multiple scenes to realize that THIS WAS ACTUALLY SUPPOSED TO BE GUINAN. I thought it was a new character that was fucking with him!
I agree that it should have been Whoopi both times -- or Whoopi and a new character (I'm not willing to give up that Whoopi scene for anything). My interpretation of her line in the premiere was that she could both age and de-age herself. I agree digitally de-aging her for these scenes would have been a mistake, but they already introduced the explanation that makes that unnecessary.
I love the idea that this could have been one of Guinan's kids. They teased us with that a lot back in Berman Trek! They kept saying one of her kids would pop up on DS9, and I was disappointed it never happened. We could have finally gotten that payoff a few decades later!