If I understands correctly, A and B will exchange their revocation key once they are ready to move to the next state. What happens if B received the revocation key and decided to close the channel unilaterally without broadcasting the transaction. This way B can never get his funds, but A can never get his funds as well. Because A can not broadcast his transaction since B has his revocation key. Isn't it a huge risk for A?
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commitment transaction, then A can close the channel by broadcast the previous commitment transaction which has not yet been revoked.