You don't have to be a member in the thieves guild to get the quarantine key - I checked this in version 1.0034. Speaking to the guard triggers Lain's speech option. If you have exposed Elrath to the authority, you won't get the key though. And there seems to be a bug. I had to speak twice to him to get the sentence where I found the key in my inventory. With this key you then can enter the section and talk to Cedric, Sara and Tibus there - maybe you'll even find empty bottles, giving you a hint that something is going wrong here.