IMO you can't expect APS themselves to foot any bill though tbh. As you said, if they sustain that they did warn all agents to use the exhaust, surely it'd be down to the agent and not the main manufacturer? have you got proof of the contrary? So no, the kit in their eyes wasn't incomplete at all. Again, it all points to the dealer and you in this case would not be able to do anything about that. So if APS does anything it would only be a goodwill gesture, but would also set a "dangerous" precendent: ie a tuner does a shite job and you go back to APS asking for compensation.
I don't know , as I sd before Giles's situation is surely difficult but at the same time you keep on trying to "accuse" everyone and anyone (APS for telling or not telling their dealers about the zorst, the dealers for selling the exhausts sparately despite being included in the kits, then APS for not coming up with some compensation because the dealer has gone bust)while really imo it's just a chain of events that made the whole issue really difficult to handle.