But players can, and do, "gainsay" the DM because the DM is a friend of theirs at their table. And can be negotiated with just like the players.Because there is no negotiation at all. The DM is right. The player cannot gainsay the DM, after all. If you, the DM, claim that the Raven Queen is X, I cannot, as a player, claim that it is Y.
Establishing an alignment restriction meets the definition of "negotiated at the outset." To use your phrasing, I see this as "You think your character is X, and I'm telling you that you seem to not be role-playing X." At which point the player should probably drop out of character and directly argue with the DM over what exactly constitutes X, since they don't seem to be on the same page any more.So, yeah, if the DM's interpretation is counter to the player's, but the DM didn't negotiate that at the outset, the DM should back off. The DM is playing bait and switch otherwise, and telling the player that he or she is playing the wrong character. I think that my character is X, you tell me, no and it's really Y. If you do so at chargen, I'll probably be fine with it, since I can choose not to play that character. But, afterwards, yeah, it's better for the DM to step back and let the player play the character that the player sees as his or her own character.
I'm beginning to lose track of your belief here. I'm not suggesting that the player should be able to tell another player what to play, but that one player's decision of what to play can be in conflict with another's. The issue is that, when the question of "should paladins be restricted to LG" is resolved, some people believe that a non-LG paladin is a non-paladin. Some of these players are DMs. I maintain that the "I think only LG paladins are paladins" crowd is not served by a "paladins can be any alignment" rule, and claiming that the second includes the first is a misunderstanding of the first group's belief. If these people are in the majority - probably not - then the game rules would be better served by accommodating their belief, just as they attempt to accommodate what the majority think a "warlock" is.But, this is somewhat besides the point. The issue was that you wanted to play a "LG Paladin" but are somehow prevented from doing so because I'm playing a CG paladin. Bringing in the DM into the equation is simply clouding the issue. Presumably, in a DM/Player situation, the DM can simply declare that all paladins are LG at chargen. Cool, no problems. It is the DM's world after all. But, if the DM allows for any alignment paladin, you, as another player, should not ever be able to tell me that I can't play my any alignment paladin just because you want to play a traditional LG paladin.