When Eren kissed Historia's hand, when he got the memory flashback, you could see lightning sparks, and I haven't personally ever seen that happen when he has flashbacks. AND, in chapter 50, when he activates the coordinate, It also has lightning effects across the other titan shifters faces! Also, don't you think its not a coincidence that seconds before he kisses her hand that he thinking about sacrificing her? If it is true, that mean that just physical interactions between Historia and Eren (wink) can activate the coordinate. I don't think that it becomes activated through just touching her, because they've spend a lot of time together, they probably held hands or something like that atleast ONCE, so if it did work like that, they would of found out. Or maybe it was because of the state of mind he was in while kissing her? I really don't know why'd they add this part in if it wasnt to introduce the fact that they don't have to kill Historia off. This part would be POINTLESS without the coordinate part. That also would help explain Eren's facial expression more, I mean those memories weren't crazy, maybe the power of coordinate made him feel overwhelmed and made his body go all weird. I donno, atleast Eren will actually get some action (wink again)
I don't know. But it's already proven that he needs to be in a certain state of mind or determined in order to transform, along with inflicting self-harm. I wouldn't be surprised if the same also needs to be true for him to activate the Progenitor Titan as well, except replace self-harm with making physical contact with a royal.
Either way, I really hope Eren at least talks this out with Historia in private. Otherwise, him keeping everything to himself could end up putting her in more danger, and not only her, but so many other people as well.