It is not possible to be possessed without giving consent to the demon. Therefore the person is responsible for the sin of allowing the demon control of himself. As for the sins that result from that first sin - well that seems like splitting hairs. Allowing a demon to possess you is mortal anyway so sins committed in a state of mortal sin can’t be dealt with until after the mortal sin is dealt with.
(This is my own personal thoughts. I do not know what the Church teaches on this but obviously I don’t think I’m in disagreement with that teaching. But I could be wrong so this is my disclaimer.)
Well, it does not matter if the sin is chalked up to Satan because his fate was sealed from the first moment of his rebellion. Making it possible for him to have “nothing to loose” and can’t be punished more than the ultimate punishment anyway. So, he can do whatever he wants and it won’t change a thing for him.
Or to a person who was mentally disabled? Or a person who was under the influence against their choosing like a person who injested a drug that was dropped in their drink?
Besides, in the case with Jesus and the possesed man, the ones who ended up “responsible” were the pigs.
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