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1.I do not known if systemsettings5 have simple way to select settings to reset from single window, but I think it should.
2. There should be login option called "Plasma5 settings reset/failsafe mode", where systemsettings5 window is loaded in new session. Plasma should not load settings until user close systemsettings5 window (in this case dialog asking of loading continuation should appear)
3. When user kill the session, on next login, plasma should show message explain what happens and why desktop was not loaded. Plasma should not load, but we do same as 2.
4. When user change some settings, we should launch message box asking to keep settings. It should use old settings, but new should been applied. When user click restore previous, previous settings will be restored - the same as Windows 95 does with resolution settings and now Plasma does.

Why? Some settings may break desktop - like setting malicious color theme. Plasma6 will probably allow to install additional window theme (button, textbox, etc. look), so it is feasible.