Double-CBC as AES-KW Substitute?
Let's assume that the final design of record structure can be altered for security and various other reasons. 


CBC mode requires a uniformly random and unique IV for each invocation with the same key. The IV for the 2nd round is satisfied with this property with the reasons mentioned in the question. 

This is basically a deterministic pesudo-random permutation on a large block, and semantic-security could (and should) be added by adding randomization inside the block. 
Since this question's been slashed, let's separate the concerns here. 
Modern ciphers such as AES are currently not known to be suspectible to KPA. Since every bit of the 2CBC ciphertext is the output from AES encryption function, such concern could be said neglegible. 
As said before, seperation of concern. 

First we should redesign the record structure, we can have 128 bytes for the site of login, 128 bytes for username, 128 bytes for password, 64 bytes for randomization. (When strings are nul-terminated, the rest of the empty area as said in the question could also be used for randomization)

These are the contents to be encrypted using 2CBC. Next we use a separate key (assuming we have a KDF for deriving encryption and authentication keys) to HMAC-SHA512 the encrypted data and append a 64-byte MAC to the end to form a full 512-byte record. 


Also remember, if you're not sure there's the need for message authentication, just have it before you find yourself having to break backword compatibility, gain should always be greater than pain.