User blog comment:Eddienam/Chapter 53 mystery/@comment-10733658-20140128103508

Theories and my take on stuff:

1. The mysterious woman is a previous Coordinate. We already have hints that Eren has some sort of ancestral or inherited memory, as he instictively knew injury was a requirement of the transformation and Grisha told him he would need to rely on "their memories". She could also be an ancestor, but it seems likely if he is experiencing something through her eyes that it is one of the "memories" Grisha told him about.

2. I'm not sure what Erwin is up to, but he's obviously not expecting to survive this final plan. That seems to be the reason behind his conversation with Nile, in which he warns him and seems to be tying up loose ends.

3. It's possible that the hardening isn't an ability all Shifters possess. Or it could be an advanced skill, one he simply lacked the information to activate. We still don't know for sure whether he really can't do it at all or simply wasn't able to figure it out on his own.

4 and 5. Both are obviously long-standing conspiracies, with generations devoted to keeping these secrets. That all those from outside the Walls have knowledge about these conspiracies suggests the Interior and the outsiders both know about each other. The way those men tried to clean up loose ends by killing Historia's mother immediately after the attack on Wall Maria shows they knew EXACTLY what was going on and were panicking.