I got confirmation today from Focusrite that in Sonar preferences > audio device, it will only show up as a single input as shown in my first post. I'm getting alternate confirmation from another Sonar user running the 2i4.
 
What this means is there is no 2nd input to use as the return for the external insert plugin. That is how it appeared and good to know that it is now confirmed. 2i2 and 2i4 users take note :)