In the "Shortcut Command" dialog that pops up when you edit a shortcut on the "Application Shortcuts" tab in the keyboard settings dialog, two of the three buttons share the same access key, which is generally undesirable. The GNOME HIG has this to say: "If duplication of access keys in a window is unavoidable, you should still refrain from duplicating the access keys for any of these buttons that appear in the same window: OK, Cancel, Close, Apply or Help." It also specifies Alt+O as the standard access key for "Open": https://developer.gnome.org/hig-book/3.10/input-keyboard.html.en#standard-shortcuts But they also show a "typical" screenshot of an "OK" button with that same access key: https://developer.gnome.org/hig-book/3.10/controls-buttons.html.en So I don't know which one should change. Maybe having it trigger "Open" would be less harmful if done by mistake, as that can always be cancelled? (Having an access key for "OK" is silly anyway, as we've had <return> for the default button (which is usually "OK") and <escape> for the "Cancel" button for as long as we've had the concept of access keys. But that seems to be the GNOME way.)
Created attachment 8829 proposed patch Access keys were removed during the transition to GTK3: https://git.xfce.org/xfce/xfce4-settings/commit/dialogs/keyboard-settings/command-dialog.c?id=838aa6f1c5344d45dde51acb31f165ccd26f666d My patch adds them back and also changes "Open" to "Browse".
Simon Steinbeiss referenced this bugreport in commit d1b799be09bc64457c26bb79b4a3c0bfcd2d0823 keyboard: Add mnemonics to add-command dialog (Bug #10495) https://git.xfce.org/xfce/xfce4-settings/commit?id=d1b799be09bc64457c26bb79b4a3c0bfcd2d0823
Almost took everything from your patch, apart from the "Browse" button. I decided to make that consistent with the autostart entry adding dialog in xfce4-session.