Sorry for your loss, but am glad to hear that you are moving forward.
I built a larger than required coop for my backyard flock because I keep their commercial feed and water in the coop. I also have grass and water in their run, but the main food and water is in the coop. I think it is important to have water available to animals 24/7. I don't know if that would have saved your birds, but it might have bought you some extra time.
In northern Minnesota where I live, we don't worry much about the temp being too hot. Even so, I know my chickens prefer to be under shade when the sun if full. Others have offered some good suggestions, but if you rebuild, might I suggest you get a remote thermometer which sends a signal to your house. I have a temp base station with remote sensors and can set the high and low alarm conditions for each sensor. When I had my baby chicks out in the attached garage early this spring, I would set my remote sensor to alarm me if the temp fell below a certain temp. With outside temps in the 20F's, I was worried about losing heat and freezing my chicks. So the alarm was just an additional check at night while I was in bed sleeping.
You can also get coop/livestock fans and have them rigged up to a temperature switch, so they turn on automatically if the coop gets too hot.
I have gone back and forth on the idea of getting an automatic door opener. For now, I have just decided to take the time to manually open the pop door each morning and shut it at sunset. That gives me enough time to check the birds for any signs of unusual conditions. Also, if the automatic door failed to shut at night, and I did not check it for myself, I am sure a racoon, skunk, mink or weasel would take the opportunity to get into the coop.
Anyway, I am the last person to throw stones about making mistakes with animals. I have always tried to learn from the experience and make changes to improve a better outcome the next time. Glad to hear that you have started over and are thinking about ways to improve your setup.
