As a mobile detailer, I often have to work in the direct sun and I am in Texas. If shade is available, I will work in it, but sometimes, I don't have a choice. I usually use
Meguiars #26 (liquid) and I have yet to have any real problems removing it even on 100 degree days. I just use a little less and don't try to cover a large area at once. I work in smaller sections so the wax won't have time to dry while I am spreading it.
If a car needs anything more than a swirl remover, then I make sure I have shade. Working paint with a cleaner or compound in the sun leads to swirls and scratching, even when working by hand.