The impression I've got is that while possession is reversible, it is not nearly as easy as that. Possession happens not as much as when you say yes as much as when you stop saying no. Which is why demons frequently decieve or torment their way into control over someone.
Presumably, the possessed can muster willpower on their own to throw the demon/spirit out though I'm not sure we've ever seen that happening. Possibly, this is what the harrowing actually entails. We've also seen Marethari holding onto her mind for a while before losing control to a demon. So it lends some credence to that theory.
But the true cause of concern is that a spirit is just an idea with a sense of agency. If I am so enraged I attract and become possessed by a rage demon, where does my rage end and the demon's begin? Initially, one might be able to tell. But after some time the lines will start to blur. Possession changes both the spirit and the possessed. In Anders, Justice turned into Vengence. A perversion based on Anders view on the injustice of the circle system. This changed them both which in the end led to disastrous results.
This, is I think, is the were crux lies with reversing possession. It changes you and it changes the spirit. Not always for the worse, not always for the better. Sometimes, like with the Avvar mage-training, this is intentional. But even then things sometimes go wrong (as we became witness to). But whether someone had the traits needed to reverse it prior to possession, they might not have afterwards. They have changed now, maybe not a lot. Or maybe so much they can never return (I speculate this is what happened to poor Niall in DAO, it was bad luck for him to face a sloth/despair demon).
I've also gotten the impression that some demon like to torment their victims, destroy their minds to such a degree they can never assert themselves enough to separate from the demon.
So in the end. Perhaps we should not ask whether possession in general can be reversed. But rather: Can this one be?