It was a fall + dementia, in my opinion
At first, I thought it was a gun related murder suicide or gas leak. Then when more news reports came out, gunshot wounds were ruled out and the gas company denied a leak. I now think:
The wife fell and died. Hackman had dementia and was confused. He eventually fell.
(less likely). Hackman was depressed and overdosed the wife. He then accidentally fell and died.
My guess is that they died at the end of January. If they have a smart meter for electricity, that could show the amount of daily use.