The only explanation I can come up with at this point for why we should lynch Ryu over Chu is that, if Chu is mafia and knew the town could potentially be at ly-lo in this phase, I don't see why he'd be asking for us to lynch him if his victory was near. I don't know, maybe it was just a ploy to try and draw us away from lynching Uni, that didn't pan out.
Other than this, I don't really see any reason not to lynch Chu at this point. Still have my suspicions of Ryu, but I agree with the general sentiment that, based on the current evidence, Chu seems the likelier mafioso. I think, having mulled over your reasoning of why they can't be both mafia, if Chu does happen to come up clean, then Ryu is likely mafia, and vice versa.
I still don't like Shish and Opo gunning for Chu right from the off; if we are at ly-lo then the last thing we need is to lynch someone on the spot, without thinking things through, but if the mafia are within touching distance of victory, then that's exactly what they'd want us to do. Even if it is someone who seems as suspicious as Chu does currently. I guess we can deal with that in the next phase, though (assuming we even survive this one).