1. It's not irrelevant if the manufacturer is different, screen size is different etc.1. Irrelevant, as it's still an example of OLED burn in
2. yeah. a lot of people do that, and leave screens on for extremely long periods of time - especially with TV's and shared screens for the family, since multiple people can use the display
3. yeah we pass 40C here, electronics do not enjoy it. 45C with a thunderstorm and bushfires was a fun, fun time.
2. Most if not all devices (especially mobile that you bring up) have some sort of screensaver or idle standby function in addition to normal OLED burn-in mitigations such as pixel shift and ABL. In order to disable the standby (if that's even possible on some devices) requires conscious decision by the user. At that point if they disable it and let it run displaying static content they have no one but themselves to blame. But it is not a normal user case.