The magic Nishina used on the rock was usable by both knights and priests. I do believe that Nishina could be a priest, there wasn't a reason for him to tell Nanaura about healing magic aside from assuaging concerns for Teshimano's safety, but that could easily have been done by keeping it vague since he knew that healing magic was a secret. The convoluted reason hinted by the story is that he foresaw his credibility would be questioned so he willingly discarded his priesthood beforehand to maintain his alias. On the other hand, Ashley Guivarch does seem to be a straightforward individual, accepting duals for honor even when he supposedly only knows weak magic and openly proclaiming allegiance to the old world royalty. My tinfoil hat theory is that Nishina actually is Ashley Guivarch, but that Ashley Guivarch also became a priest after his nobility was revoked and before he had his knighthood reinstated. His given reason for knowing healing magic's existence is plausible, they all have memory loss and it could be that he really doesn't remember everything said by his fellow miner, but I wonder if Ashley didn't feel a need to fill the power he lost with his bloodline and just gave the convenient answer that he forgot as a cover. If Ashley was a priest, it might explain why the church didn't want to give him another ceremony to learn his bloodline magic since they would have known he already had magic from two occupations or was committing some taboo by taking two jobs. Why would Eugene, a prince close to inheritance for a contested throne, keep a knight that didn't know any bloodline magic? Would blind loyalty outweigh the power bloodline magic could bring? Perhaps if blind loyalty also came with priest magic.
Or not who knows, this is such a good mystery series, so much plot and intrigue.