I have to agree on this. I have a shield tv also, but first u would like to note that version 7 does work. Allows for the floating window and drawing over apps, and yes on marshmallow.
The current version does not however, and in android tv marshmallow builds, is lacking this feature to allow the permissions. Not even in app opps.
Now your competitor, gamekiller ( not exactly sure ) i believe, latest versions also work without having to enable special permissions.
The idea of having an extra mapped key ot button would be nice. The nvidia shield tv comes with a gamepad controller, lt ls lb, rt rs rb, a,b,y,x, back button, home, play / pause button, volume +/- buttons.
So if volume buttons could be a mapped default, would be nice