I've had a dozen flights in the US that have also been overheated on the ground, probably as much as you describe. This was on several different airlines and plane types, so it happens whenever there isn't AC available for whatever reason. Sometimes it is mechanics working on or near the APU that forces the lack of A/C, and not an attempt to save money by the airline. Fwiw, most airports provide cooled air to the planes, maybe KIX lacks this facility or it wasn't working? I know HKG airport does this, and presume your hot conditions were not at HKG.

My experience with CX is that it is cool most of the time. And yes, the duvets are sometimes needed, on some flights, though I usually use them underneath to soften the seat.