Honestly, without knowing the exact situation(and I don't need to :P), that all seems very reasonable to me. At least, insofar as it doesn't seem to have escalated into a flame war or trolling effort. I can see the merit of both stances as well as what the rest of the players concluded. But if I were to look at each part individually, I'd have to agree most adamantly with the peanut gallery. 

Poster A's inability to bend their plot towards an accommodation isn't exactly the cooperative working with the group that one generally expects and implicitly agrees to when joining a group roleplay. If you aren't prepared to let your plot be influenced by outside forces, then you shouldn't really be bringing it into a public forum like that. On the other hand, if you are willing to let influences occur, but can't find a way around them to a satisfactory conclusion to the plot(which is certainly still, at the heart of the issue, yours), then I can understand trying to avoid the influences you can't work around. However, if you already have the plot idea in your head upon joining and the complete avoidance of characters (more than one of which are not yours to control) is necessary to finish said plot, you should probably either not join, change your plot, or try to talk with the other rpers before things come to a head and they can't find a good reason to have their characters ignore yours.

Poster B's inability to control their own character is... not an exceptionally valid argument. I've encountered it before, where a writer says that they are merely reporting or letting their character write through them. I can understand the sentiment because I do know that when you write a character, you are writing a person who should remain consistent with how you want to portray them. If they do not like mushrooms, you would not write them happily gorging out on mushrooms with a huge smile on their face. But it is entirely possible that they've only actually eaten mushrooms once, and they were poorly cooked, and these are, in fact, very good mushrooms and they are surprised at how good they taste. If they would not ordinarily ignore suspicious or overt, or even just curious, behaviour that catches their attention, then why would they suddenly turn a blind eye? Bribery? Distraction? Sometimes, especially if you are suddenly in a corner and did not have the chance to prepare for the necessary actions, it can seem inevitable that your character should react one way and one way alone in order to maintain consistency and continuity. If that's the case, and backtracking was not an option(given as it's a group rp, it would be hard to manage), I can understand them being stubborn and possibly, probably, frustrated by a request to change their character's actions. 

But for Poster B to say that they cannot make their character do anything different is rather silly in my eyes. They are a text based creation, and in [i]your[/i] head, [i]you[/i] are fully capable of making them  do a 180 without any consequences. They aren't going to attack you while you sleep because you made the manliest macho man dress in drag and do the hula. It might make you and everyone else cringe to see it happen, but it is not impossible to manage. And if it allows things to continue peaceably, then maybe it is in the group's better interests that you sacrifice that character's momentary integrity and work together to find a retroactive reason for why the ignoring was a plausible action. I can understand being frustrated about it though, and the stance concerning contextual reality; consistency and continuity are important to any story, breaking them should not be done without thought and preparation and maybe a back-up plan. 

I think that Poster A might be a little more in the wrong, but only if this issue came up after a lengthy period of time during which they could have been working, [u]and weren't[/u], to avoid it by discussing their plans and how to work around them with the other group members. But, along the same vein, demanding that someone's character interact with your own smacks of the sort of thing most people want to avoid. It should be a collaborative effort to entice characters into interacting together to further their and the rpers' aims ICly, not OOCly saying that this has to happen now, because it's going against one character's supposedly written in stone traits that can't be finagled by the writer to get them out of the situation without sacrificing either consistency or plotline.

As I don't know the rp, I'm not sure about the examples being brought up by the rest of the group, and can't say whether or not they're valid to the situation. I really don't know NRPs, at all, either. >.> But I should think that in any collaborative setting, working together is kind of, y'know, the key element.