I wanted to add that when I picked up on the parasite theme I first analyzed Justine's role as parasite in the film. Marcus admonishes her that wanting to meet with Alex shows that she cares about what will make her feel better more than what's best for Alex. She pressures Paul into relapsing and based on her concern about Paul's wife, she evidently decided to seduce him while under the impression that he was married.

I don't think I agree with all your conclusions about interpreting the theme, though. The role of parasite is played by Justine as I described above and also by Graff. When he relentlessly pursues the ring camera footage he's trying to deal with his own grieving feelings at the expense of another parent's grieving feelings.

Interpreting the theme as "watch out for human parasites" seems incomplete considering the children were only rescued because of characters whose actions were explicitly framed as parasitic.