If you have the mixer up in the multitrack view then switch back to edit view, it remains in the mixer view and therefore you get the message that the mixer isn't available. As it's never available in the edit view, either the mixer view shouldn't be an option here or when switching back it should automatically show the edit window rather than remaining in the mixer view.
Unaware of this issue, as I don't have Audition for Mac. But yes, if that's the case, it needs to be fixed. I was wondering about that.
Generally, the UI guidelines are to not go messing with which panels are active, but I can see this as being more useful behavior than not. In the event of a double-click on an audio file in the Files panel, bringing the Editor into the foreground if it's currently behind another panel could be useful. I'll write up a bug/feature request for review.