Tom Nook is an honest contractor. He will build you a house and let you pay for it on the honor system with no deadline.
Isabelle is the one to watch...
She finds a patsy off the train to gaslight into being the mayor. A puppet who's only role is showing up for sign ceremonies.
(edit: here she is caught on tape greeting her victim by suppressing his pleas that he is no mayor. His confusion may imply a train hijacking as well)
Presumably there was another mayor, one that disappears and is never heard from again.
Old Mayor, Tortimer knows. He's in exile, hiding on an island where she can't get to him.
Her puppy dog eyes have got you under her spell.
EDIT: The responses seem to have been focused on Nook and ignore any investigation of Isabelle. Since the release of New Horizons it's looking more and more likely that the two are in cahoots. What are they plotting in that town hall, all cooped up together 24 hours a day?