you have to think though, Isabelle and your villagers all knew what train at what time the new mayor would be coming in on... so digby, tom nook, Tortimer, anyone of these people who currently have established businesses and services couldn't be it. To have duped the five villagers and staged a scene like that? highly unlikely in such a small town.... it had to be someone none of them had seen, someone they never reference, someone who was on the train with you at the same time, which could only mean Rover!
I mean he has more insight into sarcasm, wit, and cunning than anyone else and is most believable at pulling a stunt like this. Maybe he was struggling with some mental issues since he can't remember what map to use or what time it is, and was too nervous to remember the time and day, so he decides not to face the task himself, so when he found an equally witty or nice enough person heading to the same exact place as him... he decided to let you take the fall now instead and totally play it off, especially when he notices the people outside as they pull into the station.
This person also needed to know where to send the letter so quickly the next day too. The best logical answer is Rover... besides it just being some other unintroduced character whatsoever, but wheres the fun in that?
