1.so my opinion is he is dead. or they have the body of the wrong guy that just happen to be wearing underwear with the leaders dna on it.
2.withdrawing troops aint the end of the saga. now the politicians and lawyers come in and battle it out around a table with their weapons being paper and pen.
He's likely dead, it's just that most people really don't trust the US government to be truthful that many don't want to believe it. If he really is dead then it's a bonus if fans outside Iraq/Syria still believe he's alive. If they think they are losing then they might conduct more "lone wolf" attacks.
As for what's really happening in Syria with the US and Turkey, so many theories that it's hard to keep up with. Personally I think there's been a deal between the last two and that the US would allow Turkey to grab territories to a certain extent.