Created attachment 8568 Bug screenshot. The problem is basically outlined on a screenshot. Reproduce in the following way: 1)Connect a second monitor (e.g. on a laptop). 2)Observe that there is no Alt-hotkey assigned to 'Advanced' to go an original settings screen. I think this is a bug.
Simon Steinbeiss referenced this bugreport in commit 7985880ffe136f7e0542140c60fee9b70bc7ed6b display: Add mnemonics to minimal dialog buttons (Bug #15449) https://git.xfce.org/xfce/xfce4-settings/commit?id=7985880ffe136f7e0542140c60fee9b70bc7ed6b
Unfortunately we cannot easily add mnemonics to the other monitor layout options, as those are radio buttons and not regular buttons. I however added it to the "Advanced" button.