I've always really wanted to understand why Jubelin put so much time and energy into PS. There had to be a good reason because Jubelin is a very good detective. Maybe it is time to circle back around to him?
As Lola said, and as Jubes explained in his reasoning in his books.
PS ingratiated himself into the investigation.
There was no-one to corroborate the 1½ hours in which he said he searched uphill after William disappeared.
In his wife's statement, she explained what she did that morning and did not mention what PS did or where he was.
When PS was notified that William was missing, he says he went out front and spoke with FGM. FGM says he didn't.
He suddenly decided not to go and help his dying brother that day, as planned, and instead stayed in Kendall.
When Laura Beacroft interviewed PS, he got irritated, and deflected her questions by speaking instead about his deceased wife.
Paul had a line of sight to FGM's house and said he was sitting on his back patio.
PS wasn't interviewed until 2 weeks after William disappeared, and his house wasn't searched until 3 days after William disappeared.
Jubes had a hinky feeling that PS was lying about something, but he couldn't tell what he was lying about.
(And who knows what else, if anything, the police know about PS that we don't know.)
imo