I got this idea, and now it is living in my brain rent free, so I'm sharing it with y'all. (Tim as Nagisa)
The bats realize something fishy is going on with the Japanese government, But, the government used to having to deal with hackers and the like, keeps most of their important documents on paper (in several different locations).
So the bats have to send someone over to investigate.
(Since the JL think they are just being too paranoid and reaching about the connection between the moon blowing up and the sudden increase in Japanese government communication)
Tim is the one sent, since he’s the one who has the best natural Japanese accent from visiting Japan a few times as a kid with his parents.
He gets sent to live with the sister of a JL admin member, who, while estranged, would be willing to cover for the undercover agent and act as their mother since she used to have a daughter who was set to attend the school that had all suddenly gotten a lot of messages from the government
Using pictures of her daughter, Tim is magicked into looking like the dead daughter, Nagisa.
After some light cyber stalking, reading her diary, and meeting with the mother, Tim is ready (though he hates that he has to dress feminine for what would be a long term mission).
He moves in and attends school, feeling slightly bad that he's posing as a dead girl when he is neither-but for The Mission he will do it… even when the mother seems forget he isn’t Nagisa and isnt a female
He finds out why the school has been in increased contact with the government and a reason behind the moon's explosion, though he doesn't believe that the yellow alien/meta/? is entirely at fault. There must be some sort of story as to why. He needs more information.
It goes down same as in the show with a few additions of Tim going through the officials/assassins stuff and sneaking into a few gov building solo and finds the labs + building a cure for the tentacles.
He saves their teacher and the other two who had the tentacles, but brings the under protection, leaving them with a JL communicator because he doesn't trust the Japanese government not to take advantage of trained teenagers and under new fake names.
When Tim returns, the bats don’t think anything too bad happened… until Tim stops wearing feminine clothing, walks as silent as Cass, and can stand on par with her when sparing.