Mia's mom Rhea died by accidental overdose.
It would be fitting considering the entire movie is a metaphor for the self-destructive nature of addiction, but it would also help explain a lot of the incongruencies of the story of her death as Mia is told by the spirits vs the "true" account of events.
When asked if she commit suicide by Mia, Rhea says "No, I would never do that to you", but she never explicitly mentions how she died, or implied anyone else had anything to do with it. However, if we assume Max forged the suicide note (which Rhea implies he did), he could be creating a story that masks the true nature of Rhea's death from Mia (if he sees overdose as more scandalous and harmful than suicide), while also not being untrue (even if she didn't intend it, an overdose would technically still be her own doing). It would also explain why Rhea is so desperate to
claim Riley or Rhea's soul: she still want to be a mother, and sees herself as a good one, so since Riley has already been claimed by the spirits, she's desperate to claim him as a surrogate, post-death child.The hand houses spirits of actual people who committed suicide.
Over the time, the hand itself or simply anguish corrupts their personalities and appearances further, just like Mia's mother's
Glamour Failure, compelling them to corrupt the living to join them via suicide.
Since at the end, Mia ends up bound to the hand, it implies that she did throw herself into the traffic. Eventually, she will become like the rest of the wicked spirits.
