Mū being visible to the sharingan while using Mujin Meisai is your assumption... and it's looking like a wrong one with all the evidence favoring Mū in regards to that debate. Tobirama has Hiraishin and shadow clones to combat genjutsu. Gengetsu's a high kage level shinobi, and from this range with full intel, it's not going to be a feasible task landing genjutsu.
How is that nonsense? You need to calm down. Gengetsu has several ways to attack parts of Tobi's body in the Kamui dimension, like Water Gun, 2 Guns, and Great Water Arm if he's close enough. Idk how Tobi stomps Gengetsu if he warps himself in the Kamui dimension and attempts to fight him with full intel, since Gengetsu is better than 3T Obito.
Cool, who doesn't know that Kisame is easily the best suiton user in the manga. Both Itachi and Kisame are good candidates to get atomized by Mū. So is Deidara, whose attacks can be sensed and evaded by Mū and Tobirama. Tobirama clowns Obito in similar fashion to the way Minato did.
Alright, about the Gengetsu part I (wrongly) assumed you were talking about Obito while not warped, as in while Obito himself isn't in the dimension. Stupid assumption, sorry about that. Didn't make sense for fairly obvious reasons, and I apologize for that much. You're right on that count, if Obito were to fight Gengetsu in the Kamui dimension the odds would be in his favor, if it's MS Obito. Rinnegan Obito is probably a different story, depending on how much of the Rinnegan's power Obito can be assumed to have. By feats, he probably loses.
Anyway, though, if Mu is invisible to the Sharingan, then he very obviously takes out all three/four Akatsuki members, since none of them can even see him. I guess that depends on speculation, though, and for the sake of the fight being fair I'd say Itachi can see him. If Itachi's able to see him, he can probably catch him in genjutsu eventually.
Agh, I probably came off as a little aggressive and... well, shitty. My apologies for that, too. I usually try to avoid getting obnoxious in debates with people, but I suppose I was having something of an off day. Sorry.