*Regarding Michone, I'm wondering if it's only that she had zombie guts on her that the walkers were ignoring her or as others have suggested has been infected and how isn't a target for the walkers? [indientally the zombie guts worked in season one as a way to throw them off until it rained]
It's the zombie gut thing again. Surrounding yourself with zombie odor keeps them from noticing you. That was why Michonne kept her two pet walkers around in the past. I'm surprised it took her so long to catch on this time, but I guess she was distracted.