The likelihood that the perfect crime has been committed is so low. It would have needed to be so well planned, and even then there were variables that could not have been controlled (ie. when would William be alone, when would all the neighbours be occupied and not watching, etc).
It is far more likely that certain conditions randomly came together to set the scene for an unplanned crime (although probably fantasised about and rehearsed in different ways in the perpetrator’s mind). So then Occam’s Razor comes into play.. who just happened to be there when William ran down (?) the hill on that fateful day? PS would be the most obvious and is therefore no.1 on my list. BS had reason to have been there (delivering the spare part). RD was also in the area and quite possibly could have chosen to drive/walk down Benaroon Drive given his likely close proximity and happened upon William.
But coming back to Occam’s Razor, PS is the most likely suspect. And considering the lack of anyone to confirm his alibi and his obvious inconsistencies, I can understand why he was a focus for Jubelin. IMO