I don't recall him talking about going to Yokohama. EVEN SO! How do we even know if his dad was really on the other line? Did anyone else notice how it sounded EERILY familiar to how the first convo with his dad went?
Now, our 'proof' that it isn't Kouchi, is that Mei tells us that she has the power to see the unseen. We dont' know if Mei is telling the truth. AT ALL. It's easy to believe her, because dolls are creepy, and we know of those legends about dolls being vessels for souls. Yet, why do you choose to believe this, when you choose to disbelieve EVERYTHING ELSE?
There's a lot of fishiness surrounding Koichi. A LOT. but, there's always been something weird about Akuzaw as well. And then you have how she's trying to move in on Koichi. Course, there's many other things going on which are suspicious.
The nurse doesn't recall Koichi at all (the one who died). She didnt' think he was familiar. No one else thinks he's familiar. It was only Akazawa I'm pretty sure.
Yeah, they could be wrong about the facts. It could just be some random person who has like zero interaction with the class. But, I choose to believe it's a student, to follow the whole reason the curse started.
And, while memories may be altered, I am doubting age of people is being altered. There is the man who runs the library who has been there since the beginning. Sure, he could have always been old, but then we're getting into this memory thing just being a convenient plot device that makes it pointless to even bother speculating about what's going on in the first place.
I just don't even want to speculate under those sort of terms. At that point, ANYTHING is possible and Koichi's dad could be the 'Another'. he could be calling Koichi from inside a building in the school, and never actually went to INdia. And he's not even Koichi's dad, but actually the father of Mei, and Koichi is really teh son of Reiko! WHY NOT!?
See why I don't use that sort of speculation? It just ends up becoming a 'who can imagine the most crazy scenario?' thing.