Suppose I have recorded several parts on top of each other (through the “create new part” recording mode).
How do toggle the selection between them, to open them in the Key Editor?
You could select them all and open them in the Key editor. Then you can use the ‘currently edited part’ menu to switch between them. See ‘currently edited part’ in the manual.
Or just toggle between them using the downward pointing arrow to open the selection menu (lower mid area of part) and open that alone in the Key editor.