My best guess is Aaron. He signed his death warrant when he told rick he was going no matter what unless he tied him up like last time. It's not rick, because Neegan is fascinated with him. Rick is the leader and if production of goods and services must continue, the group will need a strong leader. It's not Glenn because that's what the comics have said. You have the entire country thinking it's Glenn based on the comic, it'll be a much better surprise when it's not. Alexandria has a snitch. It's Carls "GF". She caused Maggies ills by insisting she eats the pickles. She has grown close to Carl and Maggie and now feels bad, she was the plant to force the zombie wrecking crew out the door to begin the mouse trap game with Neegan.
I wouldn't spend any time analyzing the footage to determine who received the bat. The director/creator stated that the crew was only there for the majority of the filming but none of them are aware of the person at the receiving end of the swings. We should all agree, taking 2 hard swings to the head with a barbwire wrapped baseball bat, you're not going to survive. Even if you did, you wont be worth anything to anyone.