Just my opinion, but no 5 year old can be left unsupervised for an hour, no matter how trustworthy they may be.
If mum is fast asleep in a different room, she may as well not been there at all. I know its hard being sleep deprived (Trust me, I know as I had 5 children in 10 years) but I would be too scared to go to bed, leaving my 5 year old unsupervised in another room for all sorts of reasons - from them opening the front door and going out, them answering the door to a stranger, turning on the oven or kettle, poking electric sockets and so on.
As this little one is said to be cautious, not a wanderer and not likely to leave the house on his own, I can't help but think he was taken away from the house at some point. The trail leading to the road then stopping - is that just likely to be from where he went out earlier to drop his dad off to work, and again when he came home?
Also, I've read that mum slept until 9.00am when she noticed he was watching tv in the living room, then went back to sleep until around 10 - 10.30am .... that's a very long time for a 5 year old to be unsupervised. Not sure when she fell asleep, but he could have had up to 3 hours to have got into all sorts of trouble or had an accident and been deceased before mum even woke up.
Hoping there is a happy outcome, but sadly this isn't likely after so much time passing.