Let's first go with your idea that Prompto forgets Noctis. As I mentioned before, without Noctis, Prompto would have not met most of the people he knew, so they also were removed from his memories. And with so many memories being blanked out, his mind has to come up with alternates that make sense. Enter, the MT training remnant. Or at least, what his mind would assume was the MT remnant. It would explain why his memory is fuzzy, why he was outside of Insomnia and how he survived so long.
Ooohhh...I see. So he thinks he’s an MT now? Like, fully? Does that mean he’s going to think that he has to attack when Noct eventually finds him?? Oh man oh man oh man.
Now, let's take the reverse of that scenario. Where Prompto is completely removed from people's memories. The sad thing is that Prompto's removal ... wouldn't actually affect ... anything in the game's timeline at all. But his removal is harmful, not that Prompto on the outside would notice. Because Prompto kept Noctis human. Our blonde boy was what kept Noct going for so long and to remove that? You have a king that is fully in his depression with no one able to get him out of it.
You’re right...but gods, the depression that Noct would fall into. His light isn’t there, even in his memory. He feels like something is just missing, and he’s helplessly reaching for nothing. Poor thing, nobody has ever really been able to cheer him up like Prompto has...and now Noct doesn’t even know that it’s a person he should be searching for.