That process of becoming more unlike-yourself and more like-the-other is my favorite part of the trope of a human-nonhuman pair. (There has to be a better word/phrase for describing that process)

And yeah, I think Yoshiki's mom forgetting him has to do with the mixing. Same with that moment where Maki forgot him for a moment when they first went to his house.

As for "Hikaru" becoming more human-like with his memories, I think an important symbolic example is how Kurebayashi said "Hikaru" is likely a being that exists outside the cycle of Samsara. But oddly enough when he went to The Other Side in chapter 34, he was completely at the mercy of the wheel of Samsara (at least, that's what I'm assuming that huge, circular eye creature in the sky was meant to represent). All it had him do was travel through his memories, a core component of what makes a human like you said. And they weren't Indou Hikaru's memories either, they were memories that were uniquely important to "Hikaru".

Even if it's not in the literal sense, I agree that the story is moving toward him becoming more "human-like". Even though Yoshiki insists he doesn't have to lol