As far as we know Kemmler did not release any of the inmates.
What we know of his tenure as Warden is pretty limited. We know Kemmler became the the Warden. We know the White Council spent a good portion of their fight with him just trying to prevent him from getting back to the island. (Could that have something to do with Demonreach's grudge against the Gatekeeper?)
The White Council was certainly trying to stop him from getting back for some reason. It could have been to stop him from releasing a/the prisoner/s or for something else entirely.
I think before Harry even thinks of any of this crap he needs access to Eb's journals that begin with those written by Merlin, himself.
That would likely be very helpful.
If Eb dies, would the White Council prevent, or at least try to prevent, Harry from getting his stuff now that they've accused him of being a warlock? It was the White Council who was going to ensure that LaFortier's stuff got to his beneficiaries.