Originally Posted by Pseudo Nim

Well, do you have a job / place to live / like are you basically "resident" in Japan? Do you pay taxes etc? Do you have a juminhyo? If you do, then I think that's a pretty reasonable equivalent of someone whose daily life is in Japan. Or do you come and go? Specialist - Humanities is language teacher I think, right? I'm assuming you either are attached to a school, or probably had to leave the school because of your trip back (or were in between jobs, but that's details) so you could just say that.

As far as proof, I would bring a physician's letter, and any certificates / notes you would have. For example, maybe the hospital could write something in support? (Not sure if this is permitted by privacy etc). I'm quite certain on the Japan side the specific documents aren't defined either, which creates a lot of grief on one side, but allows plenty of leeway on the other, and if your story is true (hey, don't take it the wrong way, this IS the internet after all, for all we know we're all dogs to begin with), and you have sufficient circumstantial evidence to support it (selfies with patient? remember they're date and location-stamped on most phones), then I would expect to be able to make a case on arrival.