I mentioned upthread my response to this.
If you decouple the morality and values of the gameworld from real life, then the paladin is no longer an "exemplar of everything good and true" (to quote the 2nd ed AD&D PHB). Rather, s/he is an exemplar of some fictional values that have been stipulated by the GM. Conversely, if we want the paladin to be an exemplar of the real values of goodness and truth, then unless we think the GM has some sort of special access to the content and implications of those values, we have (in my view) no reason to give the GM any special authority to judge when a player, in playing his/her paladin, has or hasn't exemplified them.
OK, I guess point by point.
Every aspect of D&D is a mere abstract of the fantasy world in which the players act out their characters through simulation in a game. These rules set boundaries to provide simple models of real world complex (real or imagined) events. No one really argues weather a sword really does d8 hit points of damage, they generally accept it as a rule to describe an event.
So absolutely I decouple the morality and values of the gameworld from real life by creating the morale boundaries as absolutes in the game world. Now, does the GM have authority to create those boundaries , either by fiat or by extension of the rules, yes s/he does, in the very way that they have over any other modeling rule in the game.
Does D&D do this well, no not really, but not because conceptually modeling the impact of morale actions, or prescence of evil, is hard to do in a roleplay, but because alignment has not be defined as a "point" based model, but a "fluff" based model.
Indeed, systems have successfully dealt with "alignment" in terms of expsoure to the corruptng force of evil, including D&D itself ("blight"), but perhaps "better" in the One Ring. These are "evil" only systems, but it is simple to concieve of a point based model across shared axis that both detail the impact of alignment scores and their in pay effect.
But the real difference here is that their is a difference between player and character action, and the two need to be treated separately. Having an "examplar of everything good and true" does not in anyway challenge the character's successes or failures in achieving to live up to those ideals.
In short, we can create a set or maorale absolutes that define morality within the context of the game, and by doing so create a framework that the relative moralty of the character can be judged.