Rather than the 'All Settings' button be disabled and at the bottom of the dialog when in settings manager, it should be hidden and then a child settings app (e.g. Appearances ) is opened, rather than disabling the search field, it can be hidden and and the 'All Settings' button would appear in its place.
Disabling vs. hiding those elements is one thing. Some will argue that disabling shows that this element exists, but is only valid in some other context. Regarding the moving of the "All Settings" button to the titlebar I will stick to what we defined on the roadmap: We go "full CSD" but not "full headerbar". https://wiki.xfce.org/releng/4.16/roadmap/general_ui/csd
Thinking about this more I see no reason to show the "All Settings" button exactly in lieu of the search button. In any case, we're not doing this now.