Okay, what I'm understanding now is that since Kanaya is neither [A] a normal human being nor [B] utterly convinced of the normality of human beings, she should be able to see the seal on Nanami's forehead after all. My sympathies really, really go out to you for even trying to understand this 6000+ page comic as an outsider.

This helps a lot and thank you for being so thorough while putting up with this questioning! I will get to actually tagging now, I promise, and might also send off a plurk friend request if that's OK with you.