Here's my take on things:
1) Use appuifw.app.exit_key_handler to assign a function that will display the previous screen to the right softkey
2) You could use uikludges to set the softkey's label to "Back", or whatever you want.
#For example, write something on the screen
canvas.text((20,20), u"Text", font="title")
#Now if you press the right softkey, you'll be "returned" to the previous screen, which was blank.
#That's because you assigned the function that shows the previous screen to the right softkey