I didn't read everything here but to get multi-channel, stuff has to be in multi-channel and use multi-channel even when in multi-channel.
Its always been a royal pain since forever which is why I bought a receiver that can upmix to 5.1 or 7.1 especially when things just won't do it and its just one setting on the receiver and I'm happy enough as it sounds great :)
Eg Firefox has to be using mutl-channel and capable of it, the website's player in Firefox has to be using multi-channel and capable of it, and the media has to be using multi-channel and capable of it. That's on top of your sound hardware, its driver, and its software, and Windows settings, and anything else such as a receiver.
Just because you put the sound device software into multi-channel or Windows into multi-channel doesn't mean you will get multi-channel in the end result to your ears.
So, if you play a game that only offers stereo for instance then you will only get stereo no matter what you do unless you use upmix such as with a receiver that offers it.
Don't know if that helps. Have fun!