A) Go into Config, Controls, Infantry. Find Adjust Stance Adjust. Hold an auxiliary mouse button and a modifier key like W. The result is Mouse Btn 5 AND W being added as seperate controls instead of Mouse Btn 5+W.
B)
1. Open {name}.arma3profile, add a value like 29 to keyAdjust. keyAdjust[]={29}; for Left Control, or keyAdjust[]={65540}; for Mouse Btn. 5 (the forward thumb button on something like a Logitech MX510/Steel Series Sensei or Mionix 7000).
2. Start the game, go into the infantry controls and unbind the 4 new stance adjust keybinds(since you cannot add Mouse Button 5+W, A, S or D).
3. Attempt to use Left Control or whatever you assigned to change stance.