Hiya!
First... TL;DR the eleven pages...just the first. So if others have commented on this, sorry to be redundant!
Second...
This is one major reason why I (we; me and my group) have stuck to the "Old Skool" style of play wherein the PLAYER IS the characters mental capabilities...at least as far as a player being 'allowed' to behave how he wants his PC to behave as well as 'solve/contemplate' things the PC's mental faculties wouldn't likely premit (based on actual mechanical stat numbers).
We *encourage* each player to RP their PC the way they want...based on what they know of their character. That includes a PC's level, race, alignment, stats, skills, etc...the whole package. Somewhere along the line...*cough* 3rd Edition *cough* ...the game switched to "You decide what you want to do in the game" to "You go by your characters mechanics to decide what to do in the game". If you had a PC with a 7 Int and the group is presented with a riddle on a door, it is the PLAYER that is going to get enjoyment from figuring it out...not the PC. The riddle isn't there to challenge the PC...its' there to challenge the PLAYER. (this is Old Skool thinking). Looking at non-Old Skool, the general consensus seems to have been switched to the idea that the PC is the one required to figure out the riddle...usually in a simple, sterile, and boring "Make a DC XX roll using skill YY or ability ZZ". Where's the fun in that?
Apply the same sort of Old Skool play style to social skills and you know where I stand; the Player should decide if his PC is actually persuaded by the other. The PLAYER of the barbarian should decide what he thinks would be fun for his character and the situation at hand (re: group). Obviously this PLAYER decided he didn't want to help the village. The PLAYER probably wanted to 'get on with' what he thought was the regular adventure, or what he wanted to do in the game (whatever it was)...not suddenly stop that to go do something some other PLAYER decided was 'more fun'. In my game, the player of the Barbarian probably would have went along with it....but likely would have had his PC start to wonder what the heck he was doing after a few hours or a day or so. I mean, his PC obviously didn't want to...but he got 'talked into it'. After a few hours/day, after being in the village and starting to help and stuff, the barbarian has one of those moments where they stop packing flour for the village miller, turn to the PC next to him and say "...uh...tell me again. Why are we doing this? I thought we were going to kill the dragon?". I mean, just because you get persuaded "in the moment" to do something, doesn't mean that once you start doing it you don't "instantly regret" it. I'm sure jails are FULL of people like this.
So, bottom line; the PLAYER does get to decide if his character is persuaded or not. That doesn't mean his PC must obey forever. All it means, in allowing said rolls to take place in the first place, is that PC X made a compelling argument and was REALLY persuasive; now, PLAYER of PC Y can decide to go along with the numbers narrative...for however long he wants....or he can just decide to ignore it. IMNSHO, if a DM is going to start allowing other PLAYERS to force other PLAYERS to play their character in some way via some skill roll....well, that kind of defeats the ENTIRE purpose of an RPG; that the PLAYER decides what his PC does in 'normal' (re: non-magical) situations. Talking is not magical...no matter how well the guy rolled on his Persuasion check.
^_^
Paul L. Ming