In prior versions of Joplin, the keyboard shortcut to open a note in external editor (Cmd-E or Ctrl-E) was available and worked whether Joplin was in Editor, Viewer, or Split mode. In 1.0.193, the keyboard shortcut no longer works, and the menu item is greyed out while in View-only layout mode. However, you can still right-click a note whilst in view layout and select Edit in External Editor.
I would suggest that the keyboard shortcut be available, and the menu item by selectable, to edit a note in external editor, in any layout view mode. I cannot think of any real downside to re-enabling this feature.