Fold, muck, and concede are not the same. Player B was not beat he was behind.
If the cards did not make it to the muck then they were not mucked.
As a dealer you are supposed to protect live cards.
Player B cannot fold - he was already in
Throwing in cards is not a dead hand as there were no more players to act
Cards did not reach the muck so they were not mucked
Even cards that reach the muck are not necessarily dead:
Cards thrown into the muck may be ruled dead. However, a hand that is
clearly identifiable may be retrieved and ruled live at management's
discretion if doing so is in the best interest of the game.
There is no provision for conceding a live hand other than at the river in a show down that I am aware of.
Texas Hold'em Tournament Rules
All-in confrontations - When two players are all in and the action is
completed, both hands must be exposed face up before the rest of the
board is run.
What is your basis for a dead hand? How would an up dead hand prevent chip dumping?