Gaslighting is a popular form of disinformation these days.
Say an untruth often enough and in a loud enough voice, the lie becomes fact (if simply to stop arguing over the inanity of it).
Cozy Mystery authors can take a cue from politicians in this.
An army of supporting characters is at the writer’s call to lay a rumor on thick and sticky. If done slyly enough, readers won’t know they’ve been gaslighted until the very, very end.
Bottom line: You’re an author. You’re allowed to lie.