Coming at this from the prospective of a dog owner, there can be a lot learned from whatever signs of the dogs' behaviors the investigators can see. Good points raised above about what they were eating such as dry food left accessible or even human food such as a pantry door left open. Also, what kind of water source was available? (I know without a doubt that mine would eventually drink from the toilet. He is a rescue that had been living in an abandoned building. We had to work at getting him to stop that when we brought him home.)
What about the potty habits of the two living dogs during the time period the two are believed to be deceased and especially around the deceased dog? Any evidence of feces or urine in the kennel will tell a bit more about that situation. Were there any claw marks on the door that was ajar or any other doors?
One horrible thought I had that I hate to even mention was wondering if maybe one of the dogs may have been over excited and accidentally knocked GH to the floor as he was trying to go outside. I suppose if that happened, an autopsy will shed some light on any fatal injuries that could have occurred in that way.
It's all very sad.