Did you start a new game when you started playing Mikio's route or did you load from a save from when only Hayato's route was available? If you could, try starting a new game and getting to the end of Mikio's route day 10. Hopefully that triggers it... Unfortunately, that's really the only thing I can think of. I've examined the coding and there's nothing that should be preventing the button from being clickable. All it requires is that you've gotten to day 10 on his route (which is the very first scene).
I think I did start a new game before, I'm not sure what happen.
So now I redownload the game and started the route to get through Mikio's day 10, turns out I missed like this whole scene! The scene from the bamboo forest to the getting hired at the restaurant. I don't know where it branched off, but the sake scene was where I started from the first play through(weird). Anyways it worked! I got the memories to work thanks.