Blake says that the fact that the unsub knows IPA (International Phonetic Alphabet) is "proof" that he can hear. The word "proof" is too strong an assertion, and a scholar with as many academic credentials as Blake has would not use it in this fashion. Knowledge of IPA only proves knowledge of IPA (although it is a strong indicator of knowledge of speech sounds) - "proof" that an individual can hear in the absence of any behavioral indicators usually has to be done with some sort of neurological other audiological testing that is not dependent on behavioral measures.
It is stated that the "Silencer" used "six-zero nylon" to suture the mouths of his victims closed. The suture shown is much larger than 6-0 nylon, which is about as thick as three human hairs, and is only available on a tiny needle. The suture used was more likely #2 silk, which is thick, braided, black, and comes on a big "CTX" needle.
The team deduces that the Cochlear implant that John Meyers received as a child is like having the volume turned up to 11 all the time. Even though it was an experimental device at the time, it is still electronic and requires a battery. The battery would have died decades ago.