I just tried turning off the Autocomplete "suggestion list" and that seems to force right click enter to actually enter instead of having to press it twice. Right click on the command line and choose the autocomplete flyout to change.
I got to the end of this string and I was disappointed to see no final answer to the question. So I am posting this answer. If you turn on "Turn on time-sensitive right-click:", it will lock the Default Mode, Edit Mode and Command Mode to whatever you set it to be, see attached. To make changes to the settings, uncheck "Turn on time-sensitive right-click:", make necessary changes then re-check "Turn on time-sensitive right-click:".
Gneuhaus you are correct however the actual problem is that of requiring a single click to progress a command:
(i.e/e.g: Press "L" then one click starts the LINE command)
Hopefully this becomes the final of this thread:
dmmeehan (/OrionHE) solution is the key:
Turn off autocomplete (you can use the settings "spanner" for your command line in AutoCAD2015) and under "InputSettings" turn it off.
This allows the first right click to become "ENTER" instead of selecting the first autocomplete and requiring a second click (or third)