Give it a few years and a few more heat waves, all the people living in old buildings which are too expensive to insulate will be getting ac. There’s no getting around the basic need for fresh air, no matter old habits and environmental costs. I’m sticking to ice packs and cold showers for now, don’t know for how much longer though.
As far as Paris is concerned, we’re actually extremely ill-prepared for high heats. Parisians have the highest risk of heat-related death in Europe. Hardware stores are packed with AC units every summer now because people are forced to start using them. Luckily we’ve had terrible weather so far this summer, fingers crossed it stays that way.
No, you must always throw the other half away.
I wouldn’t say it’s surprisingly easy. It’s possible, but it comes with substantial costs. Paris is throwing a ton of money at some of the solutions you’ve mentioned and the results are meager at best. Hopefully they’ll improve as time goes on.