Maybe Alice-who-is-really-Sophie made a hole at the back of the shed when she asked her brother to lock her in there the first time, in order to dispose of the dead body of the original Alice? Then "someone" dragged the original Alice in through the hole at the back after Alice/Sophie was locked in there on the 2nd night. That is why the body has Sam's DNA.
The pair of specs that Sophie-in-Switzerland has just thrown on the fire must belong to Lena, the 3rd girl on the rollercoaster.
Why on earth would anyone plant.....
Only 50% of middle age adult manage more than 1 brisk 10 min walk a month.
now

but if he is guilty would that fit in with other things he has possibly done?