From what I can tell you have to uninstall YouTube completely to remap it. At least I had to.
I'd like to add channel +/- buttons but I don't see an option for those functions in the button mapper app. Android(google)TV has this capability but unfortunately only TiVo Stream & smart tv's have the buttons.
"You may have paid for your logo NETFLIX, but you will not get our functionality!" - a googletv nerd dev.