One question for you, do you start steam when you boot your pc and BigBox? I have had issues with steam loading and stealing focus. I have gotten around it by task scheduler and delaying BigBox for about a minute or two. It works but the startup is a bit longer then I like.
I could load steam when you launch the first steam game but this causes the first game go back to the game select wheel, unload ahk for buttons and other things. Then it grabs focus / loads game while you have moved on. That would be fine for me but I think of guests to the cabinet and their patience / understanding.