I just had to re-read the details on your setup in the OP.on the capture card i changed it to "use device audio" and "send to desktop" ...i have game sound. The stream has sound. but that "echo" or duplicate sound persists.
The problem might be somewhere in:
"Headphones out from soundblaster to mixer."
If your headphones are going out from the PC to the mixer, then you are pretty much mixing in what you hear (game sounds) with the mic, which is being fed back to your PC.