If I know the salt, can I decrypt the commitment? Say, I salt my key(K) with y. So, my new Key is K' = Salt(K+y). Now, Alice sends commitment = E[K',M] to Bob. Bob knows the salt(y), but not K nor K'. Can Bob decrypt the message?
No, if Bob only knows the salt, he should not be able to decrypt the message.  This is based on the assumption that the key can't be derived from the salt or some combination of the cipher text and the salt.  If that is the case, the salt would not be enough to recover the key.