Re: Manipulation rolls against other PCs
It entirely depends on the players and the situation. In games where that sort of thing is to be expected, it should be allowed (and sometimes encouraged), especially when someone is playing a clever, manipulative character but is not very good at playing as such without rolling to say "yeah I totally lie my way through" or "yes my character is a very smooth operator and is totes charming."
If the player on the receiving end is getting upset over the results and don't want that sort of dynamic going on in the game (and the act isn't expected/required for the game), then that's fine, too, but I would say that if you don't want people to roll against you, you don't get to roll against them, either. So if a player says "I don't want other players to roll things like bluff against me because X" then you can, as the GM, reply, "That's okay, but now you can't do it to any player, either."
Also, sometimes people may do it to try and sabotage another player, or just because they're bored and want to stir things up, or a myriad of other reasons that don't make for good gaming, and in those circumstances they should not be allowed to do so (especially when it's against their established character/personality to behave that way!).
There's plenty more reasons for both yea and nay on the rolling/mind control stuff between players, but they are as varied as players themselves.