I'm in agreement with
@iwltfum - though I have what is normally considered high temp/high humidity (90/90, 95/80+, etc) environment, I've not raised but a handful of CX so I can't speak from much personal experience and I free ranged, not tractored (similar but not quite the same).
Based on everything I've read, the CX simply isn't a robust bird, prone to falling over for a host of often un-diagnosable reasons, with moderately high expected flock losses under even the best of circumstances - and the current record breaking weather events are nothing close to "best". Every bird is struggling in this heat, but commercial broilers are about the worst designed bird you can imagine for these conditions - only feathered legs, black coloration, and a pea comb could make things worse.
It was on that basis I determined not to raise more CX on the property, since 90/90 is rather routine here, and focus my efforts on other, better adapted, birds for my climate. (The Dark Brahma were a mistake, too!)
I think you are doing all you reasonably can. The only thing left is to take a trenching shovel and dig them a spot about 8" deep where they can bed down, spread out, and hide from the heat of the day, using the cooler temp of the earth as a heat sink. But that seems an extreme response.
Good luck making it thru the next couple days! (and chickens, even CX, are very good at hiding injuries. Its a survival thing. If you can see a limp, the injury is quite severe, unlikely to cure itself in the coming days. Hopes it makes it to cull for you!)
