I have had these items for a number of months and am trying to learn more.  I have some basic questions.

First, I understand that in order to use the 580EX II as a Slave with the 80D, the built-in flash is used as the Master.  That's fine.  Suppose I want to use the 580EX II as a Slave and attach a Rode Stereo Microphone on the 80D hot shoe.  How does this impact the built-in flash's ability to act as the Master flash?

Secondly, maybe I have really weak fingers.  The flash unit takes some effort to slide into the hot shoe as it is.  When I have it all the way forward, then there is the issue of the locking lever.  I move the lever to the right as far as I can possibly push it, but there is no "click" sound when it gets to the point where I cannot push it to the right any further.  It seems like the flash unit is tightly attached to the camera hot shoe, but I never heard any "click" sound or sensation.

I took one picture with the camera set on "Auto".  The flash unit lcd display said "ETTL", so I assume that is okay.  I will have to do a lot more experimentation before I can say any more.

Finally, I hope this ended up being a "new" post, as I am somewhat confused by the new topic posting instructions.  I apologize if it ends up being something else.