AES-128 - encrypted public domain text
if I already know what large portions of the encrypted text says when decrypted (..can it be used..) to decrypt the file?


No, beyond validating a candidate key. With modern ciphers, knowing large portions of the plaintext does not help decipher the rest. They are designed to be immune to Known Plaintext Attack, and even to Choosen Plaintext Attack. And as far as we know, AES-128 in CBC mode (as prescribed in the XML encryption standard) achieves that goal well.