To return to the original scenario:
<<<Both players report the result as a win for player B. Then, another player in the tournament points out that player A had completed a checkmating move.>>>
Both parties agree on the result and post it; A third party intervenes after the game is both completed and posted to point out the checkmate in the middle of the game.
In my tournaments, I will allow a third party (coach or parent with child present) to bring a posting error to my attention. I will research it and correct it if needed. My TD’s take scholastic results on a clipboard so a posting error is usually a TD or computer operator error.
I will not allow a third party (coach or parents) to ask me to review the actual moves in the game to prove that their kid had a win or a draw.
Does anyone allow parents or coaches to challenge the result of a game after it has been posted based on moves played in the game before the players agreed on the result? Once the word got around on that the TD’s life would be a Screaming Hell.