I'm guessing the walker that attacked Andrea in the woods that Maggie and Darryl killed was probably the the one that bit Sophia I'm guessing because Sophia was obvious bit with in the first 24 hours she was missing. This is because Otis put her in the barn and we meet Otis the second day when he shot Carl and then was with Shane at the school after. So Sophia was bit not to long after she went missing.
The barn walker woman that had her mouth blown off was Herschels wife, I know this not just because it was confirmed in the Talking Dead but when I was watching the episode when you see Herschel talking to Maggie in the kitchen the camera angle delibertly lets you see the pictures on the refridgerator and on of the pictures was Herschel and his wife so I knew that was her when she came out of the barn.
Speaking of Barn, how do you get newly captured walkers into the barn without them all trying to overwelm and attack you??