Seems wierd to me too, that along with the fact that the wife was found in the same bathroom. I thought CM poisoning effected people differently basied on weight, health and other factors which would vary the time of death.
If they were all laying in bed I would say CM poisioning makes sense but the individuals in the same bathroom is what makes me wonder about this. I could even see one person dead in the bathroom but as soon as the other person entered and saw the other laying dead in/on the shower/floor you would think they would call 911.
I believe that carbon monoxide kills by attaching to the hemoglobin in the blood, making it unable to transport oxygen.
If the son's wife heard him fall and found him unconscious, her heart rate probably soared. I wonder if the excited state could accelerate the amount of carbon monoxide she was absorbing and take her quickly to an unconscious state.
Not knowing why her husband was unconsious on the floor, she may have attempted to revive him. There are numerous instances of people rushing to rescue others who have been overcome by gas only to be overcome themselves. There are cases of multiple fatalaties resulting from an attempt to rescue one person. The middle of one's home would be a very unlikely place to suspect poisoning by gas.
I wonder if the home is of newer construction. Houses tend to be much more airtight these days to preserve energy. That increases the possibility of accumulating a poison gas.