I have an alternate.....wife drugged herself first and in his panic to get help he tripped and fell.
My reasoning....they loved their dogs. She would've turned that last dog out if she went 2nd I think.
From the standpoint of an autopsy, I wonder if they are going to be able to discern who actually died first. This scenario does make sense that if HE was the one who was incapacitated first and she needed and wanted help, she would have gotten it.
A very gruesome thought is dying alone and not being discovered right away, but I'm sure we all know of situations where it has happened. This situation where 2 have died alone and it not being, um, on purpose is truly unusual.
But whichever way it went, the dog is why I can't get away from it all being accidental no matter how implausible the sequence of events seems. Unless you make the dog part of your "plans", you don't put the dog in a situation where he can't fend for himself or where you haven't made sure that the situation will be discovered soon.