You're right, Alix wouldn't be controlled by Nicky in that way.
LOL, I think it's good that Serge limited Ella's spending. I think it 's a way of not making her spoiled. If I were Serge, I would think over-spending is too unwise, considering there were lots of poor people (however, lol, he spent much money for her jewels!)
As for the reading materials, for sure Ella felt bad about it. But I think she forgot about that restriction later on, and didn't let it affect her happiness with Serge (just an opinion, though).