I literally had to unlock his cell and go inside in order to talk to him.

I did that after I discovered he wasn't the real killer to free him (mainly just for the hell of it), but he stayed in there...an honest criminal I guess.

Mainly being the Strange Amulet remains so unless you speak to Calixto upon advice from Jorlief. If you skip this and go straight to killing Calixto, then you won't be able to loot the Necromancers Amulet.
Now, I spoke to Calixto in the early stages of the quest, but kept the Strange Amulet (still in my inventory as such

), but I was able to complete the quest. So, did I luck out or keeping it just means I can't get the Necromancer's Amulet (not really a problem on my 1st character)?