How did Paimon actually work? [SPOILERS]

I'm a bit confused about how Paimon worked in terms of possessing people.

1. What's the deal with Paimon and Charlie?
Was Charlie possessed all along?
The story seems to hint that grandma selected Charlie as Paimon's host body when she was born - but what does that mean exactly?
It did seem Charlie was susceptible to signs and signals from the supernational - but it still didn't seem she was fully possessed, she seemed like her own person.
(But perhaps her personality was a bit influenced by Paimon - as indicated by the figures she makes were similar to the trademarks of Paimon and the statue at the end)

2. How come Charlie's spirit was summoned during the seance?
I thought that when Charlie died, Paimon's spirit would be free to roam around and start possessing others.
The spell that the cult-lady gave to Annie seemed designed to summon Paimon and initiate the possession of Peter.
So why did the spell also cause Charlie to be summoned?
Is that a sign that Charlie and Paimon were the same entity? (or at least so intertwined that you couldn't summon one without the other)

3. At the end we see that demon spirits seem to be quite adept at possessing people.
Within hours they have Annie running around the ceiling, chasing Peter and slitting her throat.
So why was there even a need for the cult to make a new born Charlie into "Paimon's host body"?
If they could just summon Paimon at any time and have him possess whoever stood in his way?