Y'know what I think would be so heartbreaking?
If Ingo went to Hisui of his own free will.
Think about it: Arceus approaches the brothers with a mission. Both brothers want to go, but Ingo eventually convinces Emmet to stay behind to take care of the station and their pokemon.
So the two ask Arceus for a little time and send Ingo off with a big party. Everyone who can make it shows up, Elesa brings his favorite snacks because who knows how long Ingo will be out on this adventure.
Everyone is feeling great about this when they meet arceus again. Arceus tells Emmet that Ingo will be back like nothing happened in a few days, That's just how time travel works apparently. The two give their farewells and Ingo is taken to Hisui.
But something goes wrong. Ingo looses his memory. Emmet never realizes anything went wrong, simply powering through and expecting to see his brother safe and sound a few days later. He worries, of course, but never to the point of having a search party sent out.
He never knows that his brother doesn't remember him. At least, not until he finally comes back.
Emmet is expecting Ingo to return pretty much exactly like he left. He's met with with an older face and scars, A brother who looks at him like a stranger. Arceus is silent, doesn't offer anything but quiet apologies.
