I didn't read back to read what all I've written, but the main use for digital mixing is in high end audio consoles. These are things which typically run $10,000+ and go into studios. You can do it, but the consoles tend to be large, and the use for which you are looking at is extremely limited.
It's more of an engineering issue than anything else.
Digital audio is... well, it is digital.
If you want to mix digital audio, you need a device which recognizes the complete digital signal as an audio stream, then can introduce a mixing pattern into an existing digital stream. This typically means that a DSP will need to separate the channels, mix new channels in with the existing, equalize everything, then spit it out as a digital signal that can be recognized by whatever is connected. This is no small task.
The biggest issue remains the same. Nobody wants to do it.
Perhaps a few hundred people - maybe a couple of thousand. In the manufacturing world, that just doesn't make it a viable product at this time.
I do wish people luck in finding what they are looking for, and would love to hear feedback if you do find it. Just not holding my breath if that's okay with you all.
