I hope there aren't misunderstanding so that Yuni can claim fuuko back :v idk I hate that glasses girl. Hopefully it's just as I described it cause i wanna see more yuni abd fuuko
If the author wants to go for the happyish ending, they could go with Mashiro reevaluating her beliefs when Fuuko finds someone. Since Mashiro thinks she and Fuuko are the same, either she can decide she was wrong about them being the same or she was wrong about them being able to be loved and chosen by someone. Maybe she also realizes she did love Fuuko and was just afraid to admit to herself and risk Fuuko's rejection.
Her whole ploy could end up being weak and short lived if it begins and ends with that no one would ever choose Fuuko or herself. She doesn't seem like that much of a mastermind in reality, just someone with emotional issues.