How's this for an interpretation + possibility for sequel (Spoilers!)

Amy & Megan aren't dead. They weren't kidnapped & were neither raped nor tortured. Josh is actually a friend of Megan's neither her family nor Amy were aware of. Her meeting & abduction by him was a way of helping her escape from a neglectful & abusive home.

Amy learned of the plot after Megan's disappearance, contacted her & Josh somehow & insisted on going along, otherwise she'd blow their cover. So Amy also disappears.

All of the online business prior was to set up a plausible seduction that would convince the authorities. Filming the torture & rape & leaving it where it would be found was the final part done to show they were both dead & prevent anyone from looking for them. Megan insisted the tape be graphic as revenge against the mother that permitted her to be abused over the years. Amy had little choice but to go along.

Sequel depicts their adventures in the wider world.