One can not start the wave alone on the 15th episode of Machine Attacks. I have experienced this on the European MvM MA #4 and MvM MA #1. The lone player is basically stuck with the "waiting for players", instead of the usual "F4 = toggle ready" message. All of the three difficulties give the same result.
I am letting you know of this, because it's possible to start all of the other episodes with one player.
Thanks in advance.