Most of your comments were only possible because you are still using version 2.8 where you apparently never enabled optional (at the time) single-window mode, a

Most of your comments were only possible because you are still using version 2.8 where you apparently never enabled optional (at the time) single-window mode, and also because you previously closed the Tool Options and Layers docks for some reason. Moreover, window layout is preserved across sessions just fine, as long as I can remember myself using GIMP, which is 22 years, plus I never saw a bug report like this before (and I saw LOTS). So it's your local bug. So basically the vast majority of your problems with GIMP are self-inflicted, there's a local bug that only happens for you, and then there's the save/export separation that is, let's say, debatable. Nevertheless, thank you for going into details.