If the ball WAS ready for throw-in and you've started your count, then you can't have no call if it is dropped into the court then picked up again by the thrower, whether done unintentionally or not. If I see that the players want to change who throws it in, and I recognize that before I start my count, I'm going to wait a moment and let them switch. If the ball hits one guy's shoulder and touches the court, the throw-in hasn't begun anyway, so I'm going to wait until he secures the ball, steps out of bounds and starts the throw-in. Then I start my count. I think common sense can prevail here.
As for B1 throwing the ball in and B2 missing it, that's simply a violation and A gets the ball at the original throw-in spot, assuming no one touched the ball before it went OOB.
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