[SPOILERS] On Prompto....(Ch11 and beyond)

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So I'm on my second playthrough, at the moment. In the leadup to Chapter 11, I've been keeping an eye on Prompto to try and keep track of the switch that takes place, and any foreshadowing beforehand.

Don't think it's related, but I noticed that when you first arrive at Cartania station, he's waiting outside taking in the view while Gladio and Ignis are eating in the dining car. I'll wait for that to sink in - Prompto, renowned former chunkster, who yells 'Let's grab a bite!' whenever you pass a diner - wanders off to stare at the horizon when everyone else is eating. Kid is either so miserable he's completely lost his appetite, or he's already switched with Ardyn. Except it's clearly still Prompto after that, down in the mines, so.... yeah. A second possibility is that Ardyn is already working on him in the station but loses control when he gets into the mines, but I could be overthinking that.

I'm dubious about whether the switch has happened when they get on the train. Prompto appears to get on the train first without assisting Ignis, which is slightly out of character, but allows Noctis to step in and help after Iggy's outburst earlier. You hear Ardyn humming the Chocobo theme, but the mocking laugh afterwards sounds as though the switch hasn't happened yet. However, after Iggy and Gladio leave, Prompto comes pelting through the train, overshoots and says 'oh there you are, buddy' - wouldn't he have known where they were sitting if they'd all boarded the train together? Again, we're in the dining car. No explanation is given as to where Prompto is prior to this. Now the switch comes into play. Noct sees Ardyn in place of Prompto and chases him down. On my first playthrough, I thought that the Prompto who wakes him up is the real Prompto but now I'm not so sure - it's subtle, but the mannerisms and battle dialogue are off.

Anyway, after you warp to the tank and back to the train, there appears to be two Promptos - one in the box car who shouts 'We're leaving, come on!' - when you warp to the train, he appears to be in front of you, but then turns out to be behind on a flatbed in front of a liquid tank, who you join to battle the airships. I'm not sure if these are two separate entities or not, but I suspect the one you battle with briefly here is the real Prompto, who is then led onto the roof by Ardyn and you know the rest. I'm prepared to be wrong on this; maybe we'll find out with the DLC, who knows.

One more thing - and don't read beyond if you've not done Chapter 13 yet - do you think there's a possibility of Ardyn being able to switch with Prompto because he's an MT? I know there's controversy on the translation implications here, but hear me out. When he's tormenting Noct in 13, he only uses illusions of Prompto, not of anyone else. When the decoy collapses, he's been switched with a dead MT unit. And it was Ardyn who pushed the MTs into development in the first place (although he's not the scientist in charge - although the guidebook places the scientist in charge as Prompto's bio dad so figure that out).

An argument against this is that Ardyn generates Prompto's gun in place of a pen on the train, though. So I'm not sure.

TL;DR I think Prompto might be susceptible to Ardyn's illusions because of his lab origins - and also he's a lot more depressed in Chapter 10 than he appears.