Just completed the trilogy and although I liked it, there were a few oddities. Maybe I am remembering it incorrectly but when Quinn Dextor left Lalonde on the Lady Mac he was still Quinn Dextor, although he seemed to have possessed talking to him. Marie Skibbow recognised him as Quinn in the bar before they left.
When he was in his bunk on the Lady Mac he was tormented by the possessed but he certainly hadn't been taken over by one.
The next time we see him torturing Grant on Norfolk, he is clearly possessed as he overcomes his possessor.
So, when did the possession happen?