For all the theories thats been posted here.....
Mine would be she had to go first.......If Gene had problems,she would have called 911 asap......
Gene fell,and just like the old lady commercial," Ive fallen and I can't get up" type of deal.......
And of course the dog died of no food and water......
The door being partially opened had to do with Gene in a hurry to get to his wife......
I'd have to think the Hackman's had security cameras around the property.......Im sure someone would have been spotted.