Billboards. Just because walker is seeing konrads face on billboards doesn't mean he was dead the entire time. It means he was crazy, specifically schizophrenic, from the start.
No normal person hallucinates. Even if you have PTSD. I actually had PTSD once and had nightmares every night for several years, but I never once hallucinated. Not even when the bad things were going on. I would've liked too, but my mind doesn't disconnect from reality that easily.
Seeing things on billboards early on might've been Walker's way to associate safety with Conrad. He looks up to Conrad as a hero since he saved him in Kabul, so subconsciously, he hopes Conrad saves him again. He probably feels insecure, having crash landed on a post apocalyptic city, and since his brain disconnects, you can see early on that he's already losing focus on reality.
Now the end of him being saved is obviously crap. The CIA will pin it on him. If they took him away, he's going to gauntanimo bay.