Am i the only one thinking Bi-polar was the wrong word for Rebecca?

Thank God I'm not the only one who was thinking, "No way is Rebecca Bi-polar!" If they had said she was Schizophrenic or Sociopathy, that would have made sense.

This little girl just confessed to killing a little boy, and she wants to play. She is the Devil!

I personally think she fits Borderline Personality Disorder better than Bipolar Disorder. People with borderline personality disorder are more likely to have triggers (the fear of abandonment being a huge one) and are unable to regulate their emotions in response to the trigger, whereas people with bipolar have known their low or elevated moods to "come out of nowhere". Rebecca's moods had a source and they were a response to Sarah's actions. People with borderline personality disorder have strong reactions to behaviour within relationships and can be obsessed with people they value. They tick all the boxes you mentioned. Had Sarah behaved how Rebecca wanted her to, the perceived threat of abandonment wouldn't exist and Rebecca would have probably behaved relatively normally, which is why I don't think she was bipolar and having an episode.

Borderline and bipolar are frequently misdiagnosed as each other though, I'm not an expert and it's not like the film gave us random movie watchers enough to make a diagnosis, so I guess it's all just opinion.

I don't think the writers knew all that much about mental illness anyway. They threw in whatever the plot needed in regards to Rebecca's behaviour.