My theory is that the grandparents were desperately looking for their grandchildren who out of fear may have hidden in closets.
Also, thick smoke makes it almost impossible to see.
The terrifying accounts of spousal murder/suicide does not fit this incident. This couple were grandparents.
The most likely scenario is a gas leak which rendered the family unconscious - thereafter some other malfunction caused the fire.
The wiring and other installations in this massive building must have been very complicated - and apparently not up to code.
It’s also tough to get around in a 16,000-sq.ft. house.