Fiona and Mimi (spoilers)

Did I miss something? Why did Fiona pounce on Mimi on the dance floor? I get that she wanted to ‘do better’ than Nigel as punishment for his lusting after another woman, but was there any prior indication that she was attracted to Mimi, or even women at all?

How the hell did she know Mimi would be open to lesbianism, and with her?

From Oscar and Mimi’s perspective, what exactly was ‘the game’? They clearly wanted to seduce Nigel and then, it seems, push him away, but did they intend to involve Fiona? If she was part of their plan all along, as it appears by the end, I don’t see any evidence of this.