This is the part I don't understand 
, if the screens need to show different things then I think you will need two hdmi outputs from the Mac a splitter won't do the job (it will only copy the same data from one screen to a second screen which isn't what you want). That is what you need to find out from the folks that have done this I expect, I probably can't tell you.
Yes that is correct. That gets the 12V (in a round about way) from the barrel plug on the adapter, through the socket (powering the 2 screens on the way by) and then back in to an identical plug to that of the adapter to plug in to the Arduino. If the screens didn't need power you could just plug the plug from the adapter directly in to the Arduino which in the end is what is happening.
Peter