Plot Hole

Didn't even think of that but you're absolutely right. If Annabelle is possessed by the dead cultist then they are wrong. Unless the demon goes into the doll at the end of this film?

But then who goes into Alfre Woodard/Evelyn's body?

It wasn't the spirit of the dead girl or guy that went into the doll, she performed a ritual (hence the suicde and the marking in blood on the wall) to summon a demon. (I believe, I could be wrong. But I'm pretty sure I read that somewhere).

The detective tells Mia that after further investigation, they believe the attacks carried out by Annabelle and her boyfriend were not a sign of devotion as originally thought, but an attempt to summon something. Evelyn tells Mia that she's not experiencing a haunting by Annabelle, but a summoned inhuman spirit. The priest says the demon is using the doll as a conduit.

Annabelle was never possessing the doll, it was always a demon. It deliberately used the form of Annabelle to manipulate Mia's fears.