First, normally a message authentication function (e.g. HMAC) is used with AES-CBC, specifically to prevent the type of attack described in this question, where the ciphertext is altered by an attacker in an attempt to produce a ‘wanted plaintext’ upon decryption. See When do I need to use CBC and HMAC? for more info.
Also, the ciphertext that you posted is ...
At a high level, the Digest Algorithm would specify what algo is used to calculate the HASH eg: MD5, SHA1, SHA256, SHA384 etc.
The Signature Algorithm would specify what signing algorithm is to be used eg: ECDSA, RSA etc.
However, in most cases the DigestAlgorithm is tightly related to the Signature Algorithm so there may be duplicated information eg: the ...
You cannot fill the remaining blocks with the null bytes unless
The message has a fixed size that doesn't require sending file sizes.
The file size is sent over the channel.
The message doesn't contain null-bytes.
In the general, the padding must be unambiguous. 10..0 padding can be fine.
if data is 1234567890000000
now the data is 123456789 or ...