I don’t want to be nitpicky, but what the hey! It’s my nature!
Have you made the scale tool ATTRIBUTE visible to the user?
That would easily give you the specific indication you mention, Whoops! I’m glad I tried to test this before I finished this. There is NO option to make this attribute visible to the user!
So I’d make this suggestion (#4 from your original list) to enable visibility for the Scaletool attribute.