I have a crazy theory as to how Quinn could end up alive and not a head of cabbage. It requires reaching the conclusion that Dar is an evil puppet master and perhaps in league with Durring. He wants Quinn dead and he wants Carrie to euthenize him. Here's how I get there:
1. We are reminded a number of times that Quinn ends up at a U.S. military hospital (which in actuality is not close to Berlin at all) - there is an exterior establishing shot of the hospital which includes its name and Carrie names the hospital when she's trying to wake him up. They're not in some third world country - Germany has a fine health care system. Why isn't he at "Berlin Memorial" where they took Alison. Trauma centers at hospitals in major cities that might be the subject of a terrorist attach would be prepared to treat victims of sarin.
2. All we (or Carrie and Saul) know about Quinn's condition comes from doctors who work for the US military. A higher up at the CIA like Dar could instruct the doctors to tell a civilian (Carrie) and a lower ranking CIA agent (Saul) that Quinn has a serious hematoma and will probably die and if he lives he will have major brain damage. He looked pretty good for someone who just had hours long brain surgery (I will admit this is a common problem in TV shows).
3. Dar tells Carrie that Quinn being in a vegetative state would be his "worst nightmare" and gives her Quinn"s sad and fatalistic letter while he was still alive, which I'm sure is not protocol. Dar could easily have read the letter before he gave it to Carrie or even substituted a different letter. All we know is the envelope is the one Quinn had in the Season 4 finale.
4. Quinn is conveniently is a private room with just a pulse ox monitor. He is breathing unassisted and is not on any sort of life support - not even an IV! Carrie would have to actually kill him - no just "pull the plug".
5. If Carrie does kill Quinn and Dar knows she did it, he has major leverage over her. It would be considered murder in the US and I bet in Germany as well.
6. What is Quinn trying to say to Carrie when they wake him up? Maybe Quinn figured out Dar was up to something and is trying to tell Carrie. It doesn't seem like he would know where the attack was going to take place - the terrorists weren't discussing their plans in front of him. Also, he seemed to be saying more than "train station" which would have answered the question.
I haven't figured out why Dar would want Quinn dead and Carrie neutralized and not working for the CIA but there are a lot of possibilities. It would explain why he seemed so clueless all season (which isn't like him) and why he was in Berlin and not Langley. Dar's story about recruiting Quinn at 16 and him being pretty enough to turn heads, reveals how he basically turned a teenaged foster child into an assassin and perhaps, as has been mentioned, pimped him out! Dar has always been somewhat ruthless and unprincipled so it wouldn't be surprising. If he is somehow working with Durring, it might explain some of his odd and inconsistent behavior.
Please take potshots at these theories!!!