I suspect that part of the assumed value of the model may rest in being able to predict, with some degree of generalities and debatable reliability, where lines of conflict or incoherence may lie between people of differing motivations.
IMHO, I think that there is a potential risk here of equivocating what is meant by "conflict" in these two contexts. I agree that a general sense of conflict is necessary for both, I don't think that means that two players with different agendas necessarily see the conflict in the same manner. In the case of Gamism, conflict is competitive with optimal choices for "winning the game." In the case of Narrativism, conflict is dramatic with complicated choices for "revealing character."
I think that the two of us can look with some detachment at the former G player and see how these moments of conflict could reveal character and lead to exciting dramatic conflict. We could also look at the latter N player and see how these moments of conflict could involve winning or losing a contest with stakes involved. But that is not necessarily where the respective individuals' play priorties are in a given moment: "How can my character win this conflict?" vs. "How will this conflict change who my character is?"
So let's take a fun example of how the G and N player might conflict in a given moment of play. Here we have Elrond (Gamist) and Isildur (Narrativist):
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Elrond's player (G Agenda) recognizes here that there is an optimal strategy for winning the game right now. All Isildur has to do is throw the Ring of Power into the fires of Mount Doom to win. Game over. However, Isildur's player (N Agenda) is not concerned with the winning strategy. Instead, he thinks that this an ideal moment to test his character against the ring's corruption, curious about what that reveals about the character he is playing. Isildur fails the test and he decides against throwing the Ring of Power into the mountain, and Elrond's character gets monumentally pissed about the whole thing, holding a grudge against him for ages because he knew that they could have won the game then and there without having to wait 3000 years to finish it. Elrond's player could have chosen to push Isildur and the Ring both into the fire if he was a bigger wang-rod, but he instead decides to retire his character and let them become a NPC patron. We can read this as an illustrative moment of "incoherence."
There may have been a S-oriented player here too. And in this moment, they are likely more sympathetic to Isildur's player than Elrond's player, because of the importance of playing the character in accordance with the character concept. However, if they were playing Isildur, the S player may have not willingly tested their character against corruption or may have gotten upset with the results of a test. They may believe that it would invalidate their character concept of the valiant, noble Isildur, who would never have given in to the One Ring, because it would make their cool character look like a wang-rod. But it's also possible that another S player may have been cool with it because the game is meant to emulate Tolkien, so it's cool that Isildur succumbed to the Ring, so long as there is an easy to understand cause-and-effect regarding the Ring's corruption. But here the emphasis is not so much about testing Isildur's resolve as character as it is about making a saving throw against the corruption of the Ring as an objective reality within this world.