Hello Guillaume,
THE HAND UNVEILDED
See Rule #68: Exposing cards with action pending, including the current player when last to act, may result in a penalty but not a dead hand.
Also, there are several rules regarding tabled cards, folded cards, and verbal and non-verbal actions. The gist of which support the notion that while Player B may have acted improperly, the hand is still alive. Player A is not innocent either. IMHO, Player B wins.
THE WRONG AMOUNT
See Rule 49: Accepted Action
Poker is a game of alert, continuous observation. It is the callers responsibility to determine the correct amount of an opponents bet before calling, regardless of what is stated by others. If a caller requests a count but receives incorrect information from a dealer or player, then pushes out that amount or declares call, the caller has accepted the full correct action and is subject to the correct wager or all-in amount.
IMHO, Player B called.