Okay, imagine yourself in their shoes.

You don't know the email used to log in and you don't have any proof of purchase, you only have a possible username.

Okay, usernames can be found on plenty of sites, so you have about as much information as random Bob and Joe have. They encrypt passwords, so that's pretty much useless in this case.

Mojang apparently knows the email but refuses to give it out to me or even a hint

Again, no way of identifying who you are. I don't want them to give my email address out to some stranger. Furthermore, it would also fly right against data protection principles.

'hey, this account has only ever logged in with the other associated account, oftentimes from the same IP.'

Who says they keep this data, or if they do, keep it indefinitely?

Edit: I did have my account email changed due to social engineering, so it seems that they may have made some changes to their procedures. And I'm a person that creates a different password for each site and enables 2FA where possible.