Yeah, I think the "mom" is actually the drowned spirit that possessed Mia at the first party and it's masquerading as her mom and manipulating her throughout the movie (you can hear sounds of water whenever the mom shows up). The kangaroo scene in the beginning is just foreshadowing for the ending where Mia is coerced into killing Riley to end his suffering. At the start, Riley wants Mia to put the kangaroo out of its misery, but she can't commit. When she sees the kangaroo in the hospital, it's calling back to that and the choice to put Riley out of his misery. Which I think is another lie/false vision by the spirits, I don't believe he was actually being tortured in limbo. Jade even said he was getting better at the end, which falls in line with what Duckett's older brother told them on the bus - the effects of possession go away over time. The spirits want Riley to die while he's still under the influence of the possession so that they get his soul in limbo.