This characterisation of play seems to involve a category error: it posits that NPCs exercise causal power over the events at the gaming table.
That is impossible, though, because NPCs are purely imaginary, and imaginary beings don't exercise causal influence over real-world events.
The issue is not about "NPC agency" - it's about authorship, and how the content of the shared fiction is established.
That is, [MENTION=23751]Maxperson[/MENTION] is not "see[ing] the advisor as having an agenda that can be pushed against the players via narration" (the advisor doesn't
narrate - the advisor simply
acts). Rather, Maxperson is seeing the
GM as enjoying the authority to put into question, in some fashion, the advisor's relationship with the baron
in spite of the outcome of the skill challenge (he hasn't explained exactly how this would be done: does he imagine the GM rolling a CHA/Diplomacy/Persuasion check for the advisor against some DC also set by the GM?).
Whereas I have repeatedly, and quite clearly, asserted that
the relationship between advisor and baron is settled as part of the skill challenge. It's one of the outcomes that the players, by their play of their PCs, have secured. This is why I made the comparison to reducing an enemy to zero hp; or to a moral or loyalty check in classic D&D. These are all mechanics the produce finality in resolution: they don't simply generate temporary pertubations in the fiction which the GM is free to retest or reopen.
I think that if I had described the PCs using (say) some sort of Geas spell on the advisor, [MENTION=23751]Maxperson[/MENTION] would not be positing that the matter of the advisor's capacity to change his relationship with the baron is still open. Which is to say, Maxperson is not confused, in general, by the concept of finality in resolution. For whatever reason, though, he seems unable to accept that finality had been achieved on this occasion, via this particular mechanical procedure.
This is not correct. (And, again, appears to involve the same category error.)
The advisor had his own agenda. He pursued it. He attempted to force the PCs' hands, in two ways (at least: the session was several years ago, and so even with the benefit of an
actual play report my memory is not perfect):
(1) The advisor tried to trick/goad the PCs into revealing information about the whereabouts of the tapestry; the players dealt with this by such measures as taking steps to ensure that the most vulnerable PC, the dwarven fighter/cleric with CHA 10 and poor social skills, was not at the table with the advisor;
(2) The advisor escalated things to try and force the PCs to reveal him to the baron, so that they would be tarnished as trying to smear him and/or take advantage of his magic secrets (eg the tapestry), rather than being the heroes who had saved the baron from his influence. Had the players failed at the challenge, it is likely that something of this sort would have happened, as the evening would have ended with the advisor leaving the dinner, apparently forced out by the PCs speaking against him but not proving their case.
His attempts to do this, however, failed. The PCs kept the information about the tapestry secret; and the advisor's strategy (2) ended up backfiring, as the baron accepted the word of the dwarven "paladin", in the context of the advisor being goaded into revealing himself.
Just as, in combat, a NPC can't keep pursuing his/her agenda if reduced to zero hp, or if his/her moral breaks; so likewise in a skill challenge. If the challenge is resolved with the players successful, and their success defeats the NPC's agenda, then that is as it is: the agenda has failed.
In my view this doesn't mark out any meaningful distinction.
There is no meaningful difference that I can see between
the world exists for players to pit themselves against, as an antagonist (which is your characterisation of "player-centric") and
the world and NPCs have their own agendas, that they force against players (which is your characterisation of "GM-centric"). Both describe relationships of opposition between the desires of PCs and the desires of NPCs/antagonists (or "the gameworld" in some more general, somewhat anthropomorphised, conception).
The difference, rather, is over who has the capacity to
resolve those conflicts. Can the players do so, via action resolution procedures? Or do conflicts continue on until, on whatever basis, the GM deems them to be exhausted as the subject-matter of play. [MENTION=23751]Maxperson[/MENTION] and [MENTION=29398]Lanefan[/MENTION] appear to be expressing the latter preference.
That preference may lead to fun RPGing, or not, depending on a range of factors including (perhaps most importantly) the preferences of the participants. But it seems pretty clear to me that it is not a preference about the "agency" of NPCs/antagonists, or their capacity to oppose the PCs and thereby to be a source of opposition to the players. It is a preference about who exercises what sort of control over the content of the fiction, and be what sorts of procedures that control is exercised.