She does know how to control her soot actually. Its just she never tested her max capabilities of her power until emilyko help give her ideas how to use her soot in many different situation.
She does now, but I am talking about all the way from the very beginning where she first realize that she can control her soot because Emilyko's plushie "jump" for the first time.
I think that’s something only taught in the shadow house which she wasn’t aware of. Just a theory but it’s prob something she didn’t need and so u wouldn’t discover those powers unless ur in specific situations.
The shadow house is meant for the shadows to grow and unlock their powers
Also another thing is she’s still in her kid form so it’s expected she doesn’t know a lot.
Wait hold on. If Kate is someone of intelligent who had already exist before meeting Emilyko, who obviously had a some sort of goal to take back whatever this Mirror House is, how does she NOT know that she can control soot until neat the Debutante?
The story made Kate out to be an intelligent girl who was born to suspect the Shadow House, but now she apparently someone of pre-existing before the plot, sound like a big plot hole.