One factor I think we need to consider with Rita and Mashiro. It's a factor that Zavie rightly brought up several pages back, but since that it hasn't been discussed much.
Mashiro... isn't exactly a normal teenaged girl. And I don't just mean her talent for painting. I mean that she seems to have
Savant syndrome, or something similar to it at least.
Mashiro doesn't speak her mind as clearly and thoroughly as what most people do. And it's not a matter of shyness - Mashiro is very bold if anything - But it's rather a case of Mashiro seeming to lack the capacity for detailed conversation.
Rita probably sees Mashiro as somebody who can't be trusted to look after herself. And given what Mashiro was like at the start of this anime, I don't see how you can blame Rita for that. Mashiro really did have
major issues with looking after herself at first.
I wouldn't be surprised if back in England, Mashiro was diagnosed as autistic, or something along the same lines. And so she was babied not out of some malicious desire to control her, but because people sincerely thought that Mashiro just lacked the mental capacity to learn to take care of herself.
Rita perhaps doubts that Mashiro has the mental capacity to really think about the choices put in front of her here, and hence takes a more forceful stance. As someone who has an autistic cousin, I can certainly understand why Rita would do this.