The much simpler answer I give to my trainees on leadership courses is Cpl/OCdt Bloggins is section senior and derives that authority from the person who put him/her in charge, i.e me.
Generally speaking, few candidates will mess with the appointed person in charge because for all practical purposes that means messing with me.
Cpl A has been appointed into a leadership position by someone in the CoC, and derives his authority from the CoC. If Cpl B or anyone else wants to complain about the form of the briefing, there are various approved means to do so. If they choose to go outside those bounds, then they will be on the receiving end of a Sgt or WO's jacking/disciplinary action, and for good reason.