I hear you. Ours are family too. It sounds like you're doing all you can - I'd just watch that mister and how saturated in makes the ground. Maybe try misting heads that create a finer spray pattern? If you can put fans in their run during the day, that could help too.
FWIW, we've brought birds into the house (into the A/C) when we feel like they might be really overheated. If that's an option, do it. They will appreciate the break from the heat.
We've even considered a window A/C unit in our coop because it's in direct North Carolina sun most of the day - haven't had to go that route yet, but it's an option.
One last thing: if you have a garage that you could cordon off a section for them, that might be an option with a window A/C unit. I'm not wise enough to know what 115 degree heat feels like on a sustained basis, but it MUST be miserable, and I'm sure the chickens are not dealing with it well... :-(