Americans aren't cooling their homes to 64F in the summer, I have no idea where you get that idea. Generally people will cool their homes to somewhere around 72-78F, which is 22.22-25.56C.
I do at night, just for the bedroom. Need it to sleep well. I Keep it around 75-78 in the house in the daytime, on hot days, and just dress for it, no problem then.
I've lived in a house with no AC and even slept outside on a hammock on the hottest days (luckily that was a house built in the 19-teens so it could cope with AC-less heat better than modern houses, but still, on nights when your nighttime lows were still in the high 70s and the air wasn't moving, outdoors was cooler than indoors) and I didn't acclimate, I just only got like 3 hours of sleep a night when it was hot, every single time it was hot.
Need the cold for sleep. I'd have to move farther north for my own sanity if not for AC (and I'm already far enough north I only need AC to achieve low-60s nighttime temps for 60-70 nights per year). I could lay off it a little if work didn't still expect me to function when it's hot, but they do.
It's hard to accurately generalize an entire country, of course. But in my experience people generally are in the range I stated. I'm sure that there exist people who turn their AC down below 70F, but I've never personally encountered one. And even the low end of my range, 72F, is uncommon in my experience.
In my experience they absolutely do. The houses by and large aren't very well designed or insulated, and electricity isn't that cheap. In the southeast I rarely see home thermostats set to the 60s or low 70s. Office settings are a different question, though.
I've lived in a house with no AC and even slept outside on a hammock on the hottest days (luckily that was a house built in the 19-teens so it could cope with AC-less heat better than modern houses, but still, on nights when your nighttime lows were still in the high 70s and the air wasn't moving, outdoors was cooler than indoors) and I didn't acclimate, I just only got like 3 hours of sleep a night when it was hot, every single time it was hot.
Need the cold for sleep. I'd have to move farther north for my own sanity if not for AC (and I'm already far enough north I only need AC to achieve low-60s nighttime temps for 60-70 nights per year). I could lay off it a little if work didn't still expect me to function when it's hot, but they do.