So the mixer plan has been aborted. The ART mixer I bought turns out to be "dual mono" not stereo. This means it has stereo inputs which it combines, runs through a mono mixing path and outputs to two mono connectors wired together. I can't begin to imagine why.
I picked up another mixer from a nearby Maplins but it's physically large, and I think it may be mic level inputs (even though the spec sheet doesn't say as much). So, with my car sat around in pieces and installers looking irritated, I've abandoned this part of the install. The 3.5mm jack will connect to the empeg aux input, and the car kit and nav unit will have their own speakers behind the dash. Maybe I can do it properly another time.
Overall it's still going to rock though! Should be ready tomorrow