Yeah your correct but the MC's character says at one time he grew up with love and he's got more to give than one can handle and all that so going off that fact such a person would try to explain the reason she never has to call someone that again. Character wise she wants to be their maid because she feels indebted as they helped her out but also she still has a slave mentally where she believes that she must prove her worth or they will send her away, hence my opinion on having Nana and MC having a moral code to change that mindset towards a family orientated relationship e.g. Big bro type of thing instead of a Master and subservient type of relationship. Sorry as I said just my opinion and just my moral code towards that. Anyway we got a new update to play which will be great and as far as team Amy.....HELL YEAH sooner than later please! and let Nana step up to the plate and bat! the MC's already pitching.....
I think you are overlooking the fact of how incredibly uncomfortable it would be to Shizuka not to have some kind of subservient role, it's all she has known and they are changing so much plus what kind of president would it set to her to be like "your free to do what you want or say what you want" then turn around and be like "No you can't do that or that and don't say that!".
She has not been there a week yet but they have made progress with her, look at her taking advantage of her licence, she is exhibiting free will, they let her use master but use other terms for her when referring to MC and more importantly refer to her with terms of endearment.
The sudden change you want her to go through would require some kind of great personal sacrifice from Nana or MC and would generally need to be something that unexpectedly happens for her to see the actual difference to make her go through a crisis of identity.
I'm sure MC would not mind one bit if she "beat" it.