I don't get the story. Can someone tell me how Miki-chan didn't realize that wasn't her childhood friend? Like the MC was black-haired, her childhood friend was not??
I think it's because Miki didn't care about her childhood friend. That line about focusing on the future rather than the past really hits different after you finish the story. At first I thought it was romantic. Now it feels...painful. Miki willingly threw out her old photos. I thought that was going to be explained by a flashback of childhood angst but it wasn't. Miki also didn't know that her childhood friend liked her, said friend wasn't willing to confront her up until she died. So all in all, I don't think the childhood friend meant as much to Miki as Miki did to the mc or the friend herself. And because Miki didn't care, she let herself get fooled.
Another reason why she was fooled, the mc was a good actor. I can't tell whether the mc legitimately thought she was worthy of the childhood best friend role, or if she was purposefully trying to distract Miki from her lies by making her think her memory was wrong. But either way, it was effective. Especially considering the Miki didn't really care about her childhood friend.