- Except most of the mirror's magic is illusions, which aren't filmed. Even if they reviewed the tapes, it would just look like the siblings going crazy (moving plants/cameras then being surprised at it happening, standing in place for no apparent reason, freaking out at things that don't seem to be there, etc.). So Tim will probably go back to an insane asylum, not a prison and the mirror will be go to whoever bought it.
- Maybe reviewing of that video evidence will strongly suggest that there's something peculiar about the mirror, and perhaps a way to destroy it can be found after all through a concerted, government funded research effort. Well, I can dream, at least. I like happy endings.
- Beyond that, the few physical manifestations are impressive enough to keep people who are fascinated with the macabre interested in the mirror, which is probably just the sort of thing that it wants. Lights going out and plants dying wouldn't exonerate someone for a murder since they obviously wouldn't prove that something supernatural was going on (extraordinary claims require extraordinary evidence, and such), but they would draw in paranormal buffs and people who think that they can outsmart the mirror. It's been around since before the translation of the King James Bible, definitely a long enough time to develop some skill with long-term planning.
- It may have been the delusions of only one of the siblings, probably Kaylie. Likely, Tim wasn't even there for the second half of the film. He got fed up with the mirror nonsense and split, leaving Kaylie all alone and allowing her to sink deeper and deeper into the throes of insanity. At the height of her paranoia and hallucinations, she killed herself. Tim's being taken into custody while screaming about the mirror occurred only in Kaylie's dying mind.
- And the events of the film will be enough to attract the attention of the Foundation. They will step in and take possession of the mirror. It will probably be classified Euclid due to the full extent of its abilities being unknown, but it could be downgraded to Safe after being studied. The containment procedures will, ironically enough, be partially based on Kaylie's methods for dealing with the mirror. They will also take custody of Tim and, after interviewing him about what happened, either offer him a job as an agent guarding the mirror, or simply give him a Class-A amnestic and let him go.
Fortunately, once the local Aurors get wind of what happened, they could clear up Tim's situation and presumably give him a Memory Charm so he could live a normal life. As for the mirror, they could probably destroy it fairly easily:
- Hire some Muggles to move the thing somewhere spacious and isolated, like an empty building in the middle of nowhere, or a big boat on the open ocean. Don't tell them why they're moving it, of course — not just to uphold the Masquerade, but also to keep the mirror from figuring out what's going on by reading their thoughts.
- Get all the Muggles out of the way — by road if the mirror is in an isolated building, or by airlift if it's on a boat.
- While staying outside the mirror's thirty-foot sphere of influence, torch the building or boat or whatever with Fiendfyre, then fly or Apparate to safety as the Fiendfyre engulfs the mirror.
That, or just shoot it from a safe distance with a projectile coated in basilisk venom, if there's any basilisk venom on hand. Or figure out a way to launch the Sword of Gryffindor at it from at least thirty feet away.
True, according to the original story, that mirror got smashed into a million pieces, but maybe that whats the mirror wanted everyone to think...