Well Rio seemed seriously distressed about killing her, which was strange in the first place. Why would she mush her friend / almost lover like that? But she'd be one hell of an actress since she literal threw her fight in part because of that despair. Maybe we get a proper explanation but right now it smells like deus ex monstergirl.
As I've said, the first theory is almost not possible but still a possibility. Who knows?
Yeah I thought maybe Rio was in on it, but she seemed pretty fucking sure that she killed Mai as she was fighting Pure, so I don't think it was a plan. I think the answer is that the gym floor was hollow underneath and she fell into it. Afterwards the Sons had her stitched back up before sending her back out to go kill Yashin.
Who knows? Maybe they had it planned, but the fight was still personal (On Mai's side, at least) with no regrets about the outcome. In KB it seems that even friends don't have a problem fighting each other to death, and some characters here might find disrespectful if you don't fight with all you got.
Personally, I think she had as primary goal defeating Rio because of their backstory, as it seemed like it was personal and I think she would have preffered dying fighting, but it seems she just got brutally knocked out.
