J.S.
I am so sorry for your loss. I couldn't imagine...We used the Angelcare monitors with all 3 of my kids for the first year of their life. Fortunately we never had to test them for stopping breathing. When I would forget to turn it off and picked them up, the monitor would always go off. The only problem was as they got older and moved all over the bed, they would get off the sensor and it would occassionally go off. I'm not sure how well it would work with an older child in a bigger bed. I can definately understand your fear with your younger child and if it were me, would definately look into some sort of monitor. You might want to look around on -line, or even check with your doctor about some sort of medical device to use for your 2 year old.
I pray for your entire family and that you find the answers about your older son that you are looking for.