To go with the last idea, the last thing Emmet saw of his brother was the video Ingo was recording for him as he explored the Icelands of Hisui. He was watching it live and saw the Zoroark attack as it happened. He frantically texted Ingo after the recording stopped and even tried to call, but the call function for the Arc Phone was disabled and Ingo was long gone by then anyway. He then tried and failed to contact Arceus, but to no avail.
He was absolutely inconsolable for MONTHS afterward, believing he had just witnessed his brother being murdered. What brought him out of it was when he finally revealed what happened to Elesa, who insisted he tell her why he was so sure Ingo was dead. He told her about their meeting with Arceus and showed her the phone and video. She is horrified at first but then catches the sound of Ingo’s fleeing footsteps at the end of the video, proving that Ingo managed to get away. And while the strange Zoroark was covered in red splotches, none of them seemed to be fresh blood, meaning that Ingo probably got away relatively uninjured as well. There is still a very good chance Ingo is alright, and simply hasn’t been able recover his Arc Phone due to it being lost in the territory of a dangerous pokemon.
They contact Drayden, the professors, EVERYONE they can think of to review the footage and find a way to find Ingo again, but even after 5 long years little headway is made since Time Travel is something only the most powerful legendary pokemon can achieve, and the legendary pokemon are as elusive as ever. Not that time travel alone would help them, since unbeknownst to them Ingo is also in a whole parallel world to them, not in the past of their own world.
I also have such a nasty evil plan involving a Zoroark later in the story. I mentioned before that I had the idea of an enraged Drayden attacking the MC because he thinks they are a Zoroark, and when it gets ‘proven’ he damn near kills her. Seeing that last video of Ingo has given his family serious PTSD as far as Zoroark are concerned, and Drayden will not tolerate a Zoroark impersonating his nephew and literally toying with their emotions.