The octet string AL is equal to the number of bits in the additional authenticated data A expressed as a 64-bit unsigned big endian integer.
The ciphertext is used to compute the authentication tag, not the plaintext. JWA 5.2.2.1.
The encryption key size (CMK) should equal the digest size here, that being 32, 48 and 64 octets for SHA256, SHA384 and SHA512, respectively. JWA 5.2.3, 5.2.4 and 5.2.5.
For the three variants of AES_CBC_HMAC_SHA2, the encryption and MAC keys are always derived the same: the first half of the input key is the MAC key, and the second half is the encryption key. JWA 5.2.2.1.