Seat change questions

[redfisher]

Player A has a seat change button in the big blind. Player B busts and Player C is called to the game. Player A indicates he wants the seat vacated by Player B. Does Player A have to move immediately or does he get to play through his button and hold Player C out of the game for 2 hands?

Second question. In a must move chain, if Player A gets to play his button, should the brush make sure that no player in the main game wants the vacant seat before pulling Player C out of his seat in the must move and making him stand around for 2 hands he could otherwise be playing?