So I was recently re-reading SmF, and something new struck me from Harry's final conversation with Nicodemus on the boat. Nic says it's ironic that Harry, despite being an opponent, is the only one he can be sure hasn't gone over to the "Black Council", as Harry is still calling it at the time. And when Harry asks why, he says because "Nobody so obstreporous has ever been corrupted except by his own mule-headedness".
Now, despite not filling Harry in on the details, Nic has pretty clearly recognized that what Harry is calling the Black Council actually means he's started connecting the movements of Nemesis' infected and their pawns. There are two possible interpretations - either he's being honest about his confidence that Harry hasn't been infected against his will, or he has nothing to fear from tipping his hand to knowing more than Harry does about this faction even if Harry had been forcibly Nemfected (which the Mothers later told Harry amounts to painting a target on yourself to be killed or forcibly turned), meaning the Adversary already knows he's an active opponent and he has nothing to lose talking to Harry about it even if Harry is Nemfected.
I think at minimum, this is the hard proof that Nic is sincerely opposed to Nemesis. Even if he was for some reason lying earlier on about being disturbed that Harry vouched for someone with Hellfire being in on the Adversary's attack on Arctis Tor, the later comments only make sense if he's genuinely set against Nemesis.
But it's his confidence that Harry hasn't been forcibly turned that's perhaps even more interesting. It can't really be because he considers Harry too stubborn to corrupt, because according to his plans, Lasciel's shadow is supposed to be slowly working on doing exactly that. So that reason has to be a deflection. Unless he just figures he has nothing to lose talking relatively freely to Harry even if he is Nemfected, what does he know that makes him so confident? Are Starborn immune and he knows Harry is one? Are humans in general immune, or immune unless they collaborate willingly?