It's possible she passed away from something medical (heart attack, stroke, etc.) while in the bathroom and he wandered around and fell - I haven't heard anything about him in years, so I don't know if he was still mentally sound or if he had dementia or Alzheimer's or anything like that.
Carbon monoxide is still a possibility - I see the reports saying the door was open, but if I had a nickel for every time someone on scene told me a door was open when what they really meant was it was unlocked, well, I would have a lot of nickels. There being a doggie door where the other two dogs could go in and out is probably what saved them. I had a similar case many many years ago, where a couple were both drug addicts and got hold of a bad batch. They both overdosed and weren't found for about a month. Their dog had died; there was plenty of food in the house but the poor thing ran out of water. If only he had known to flush the toilet to bring more water in. :(
The space heater near her could be why there were more signs of decomposition, though I read that his showed signs as well, no word on if it was as pronounced.
