In reply to Dan.

Hi Dan,
actually I did that when Loopback was still in beta, I should really do a writeup about that.
The sad thing is that you still need a bunch of virtual sound cards (created by Loopback, but a bunch of them nonetheless) to handle the audio going back to Skype, while you can use Loopback's routing abilities to have a single sound card that gets the audio from all your instances of Skype and maps them to a multi-channel output (screenshot here).
Then you still need to join all of them using an aggregate audio device.

Loopbacks makes things a little better, but sadly it's not the definitive solution I hoped it would be.