Lightning networks revocable commitments: unilaterally closing could cause funds lost forever?
What happens if B received the revocation key and decided to close the channel unilaterally without broadcasting the transaction


By definition, channels are only closed once a commitment transaction or the closing transaction is broadcast. So B cannot close the channel; he simply withholds data from A.



Your scenario is not possible because A will only give B the revocation key once both B has given A his new commitment transaction. So if B refuses to give A the revocation key to his commitment transaction, then A can close the channel by broadcast the previous commitment transaction which has not yet been revoked.