I believe Jeremy was a regular smoker so its probably how he relaxed but even if he didnt hed still find it difficult to wake in such circumstances. Also why choose to stay up all night and murder your family when your knackered from 3or4 days harvesting. I would have thought that was completely irrational.
My belief is that had it been Jeremy,,he'd have been shot,,either as a frightener,or as an intention to wound. Either Nevill,,or even Sheila would have done so,on hearing a " noise ",or the dog barking would have alerted one of them that someone was around.
So saying,,as Maggie has already said,,that Jeremy didn't have a mark on his body,,as said by JM,,,and she'd have been the first to report if there had been.