An educated guess might be part of it, yeah.
Interesting, though, that Nic is apparently totally confident in his read on Harry, while Rashid was actively suspicious in the other direction until at least a book later.
I think Nic knows Dresden better than Rashid.
To your question regarding his sincerity, I think he genuinely aims to save the universe (after his own fashion). Also, this seemed to already been known by the adversary given the apparent infiltration of his circle.
Whether his reaction to Dresden at the fish house was real or a show to distract and gauge Dresden’s reaction (perhaps to determine if Dresden was an ageNt), I’m uncertain. Did they even intend to kill Dresden, either at the fish house during the abduction or later on the island? They seemed to give it a good effort, though, again, I’m unclear if Nic willed it or not. However, they also, apparently, laboured under the misapprehension that Dresden was, at some level, one of them.
So, I think Nic’s admission to Dresden was neither daring nor dangerous. Nic was already targeted, therefore, speaking openly to Dresden was no risk as he either was an agent of N taunting Nic to his face or not. Whichever it was didn’t matter as Nic was already exposed.