Some questions I'd have, either the therapist could answer, or if I felt comfortable enough to give it a few sessions, maybe I could see for myself how things unfold. If the T becomes more skilled by dealing with N victims as well as Narcs, then that is a good thing, right?
I'd ask the therapist what they think they learn or gain from engaging in a T/client relationship with a Narc. Do they come away with more insight? Do they see common patterns? And do they realize when they're being manipulated? Because if there's a Narc in the room, then someone's ALWAYS being played at some point.
And to your point about the main trait of Narcs (they never believe that they're the problem, it's everyone else that's the problem), I'd ask how a therapist approaches treatment with someone with this attitude. Is it more of a "life skills" kind of thing, where the Narc learns how to behave more decently in a superficial way, but otherwise remains unchanged? Or....???