I don't see why there would be a riot. He'll probably complain about something and make an excuse, say it was down to PED's or something but I can't see a riot like there was after Bowe/Golota.
Depends how he loses. If it's in a close fight, then expect him to do what almost all fighters do when they lose close fights and complain.
I don't see Mayweather fans on sites like this being "followers". The way things have developed, they have grown into more "anti-Pacquiao" fans. If Mayweather lost, I'd expect to see a very small few clinging onto some delusional idea of what Mayweather won the fight, an even smaller few giving respect to Pacquiao and moving on, and then the majority moving on to say Pacquiao is ducking someone or he is too scared to fight X. Mayweather wouldn't do much in 2015. He's not the same ****head anymore. Mayweather seems like a much more level headed guy and has done so since he went inside. He probably looks at Broner and is embarrassed that the guy is basically emulating him. If Mayweather lost to Pacquiao, I'd imagine that he'd raise his hand in the ring and call him the greatest of all time. Pacquiao fans would switch in a second and love Mayweather after it and the site would be a lot more peaceful. If Mayweather wins, Pacquiao fans are going to morph into Mayweather "haters" and will talk down on him ridiculously, citing that he has no "heart" or something like Pacquiao did.