I breadboarded both of the solutions today. Both seemed to work, but the very simple version I posted actually had quite a bit of very high frequency noise at the output of the opamp buffer after the mixing pot. Aside from that, it worked fine and sounded fine
The proper solution (figure 7, Nigel) also worked fine and without any of the high frequency noise on the output. The way both of them mixed sounded quite different though, but neither sounded better to be honest.