You haven't really answered my question - if alignment is not relevant as a guide in the situation @Herschel described, when is it relevant? What role does it play?
The term "guide" and "straightjacket" are not synonyms. I would expect any player's decisions to be guided by alignment where it exists, and any other personality traits, aspects, beliefs, or whatever description the game, or the player, uses. That does not mean there is commonly only one right answer. Typically, when there is, other answers don't even occur ("My Paladin tears the baby from its mother's grasp and hurls it over the parapet", for example). Much of the best role playing occurs where no clear-cut answer exists, and we are faced with which of our morals and beliefs must be compromised to serve the others.
I have not seen many cases where the GM and players have fought over alignment-appropriate behaviour. I also haven't seen too many people try to Bluff a creature with no INT, yet the skill description still feels the need to tell me how to adjudicate this.
Also, if the players are free to make their own choices 90% of the time, why is it so important that the GM judge them the other 10% of the time? And why should the players of those LG PCs be penalisd (eg by XP lost, as AD&D suggests in both editions) because they decide that on this occasion the sellsword is right?
Please show me in the various cited passages any statement that suggests a single action that seems (or even blatantly is) inconsistent with the stated alignment results in an immediate alignment change and loss of a level. I see a lot that refer to a pattern of decisions which indicates the alignment being played is not the one on the sheet, and none which match the "one decision spells doom" straw man you seem to perceive. The Paladin's "knowingly performs an evil action" rule is specific to the Paladin, and is much more harsh, but "knowingly" sets the bar a bit higher than "makes one decision a bit off the beaten path".
No it doesn't. CE, LE and NE end with a description of why they're the worst.
It does stop saying "best". It does not say "worst", but "most dangerous". And given evil is about hurting, oppressing and killing others, I'd say "most dangerous" is the best. But I did misrecall - only the non-evil alignments are treated equally with why they are "the best". Perhaps this reflects the effort of WoTC to steer the game away from bad press, as I believe 3e was not encouraging of evil characters in general.
There's a reason that REH's Conan stories have no clerics in the D&D-style, only various sorts of sorcerers.
The wily rogue, the stalwart warrior and the devious wizard all seem much more Fantasy tropes, pre D&D, than the Cleric. REH was pretty short of heroic spellcasters at all.
I've posted multiple times - the fictional events are not in and of themselves the great gaming moment. What would be an impediment to my participation in that gaming moment would not be alignment preventing the PC sacrificing his friend and companion - who thinks alignment prevents players declaring actions? - but that it would require me to judge the action and potentially impose a new ingame state on the PC as part of my refereeing responsibilities. And of course the player would know I was doing this, and respond appropriately to that knowledge in his playing of his PC.
I again come back to the belief that playing in character will only happen if it is rewarded. So the character loses a level? As has been noted above, that often happens if he fights Undead creatures. He's still viable and playable. 2e moderated this to only occurring when it was not good for the game, acknowledging that, sometimes, good role playing leads to a change of alignment.
But if I choose to play an Honourable character, whether in a game with or without alignment, with or without mechanics to enforce my Honourable play, then I made that choice because I wanted to play such a character, with the trials and tribulations it entails. I don't have to have a suite of powers and bonuses to reward honourable play. I expect there to be both benefits and costs to my character choice - sometimes, that means not taking the easy approach, and suffering disadvantages, temporary or permanent. That's part of what I chose to play. I have no desire to join your game where an honourable man is only honourable because the game mechanics favour his honour.
The difference between the god and the angel is that the player is free, in character, to judge the angel wrong. This is an example of those "physical" moral consequenes that @Manbearcat described about half-a-dozen posts upthread.
In my view, angelic lore suggests the Angel acts as a true servant of the deity, and is no more likely to be "wrong" than that deity is. YMMV
Would play have benefited if I had contracted the possible outcomes of the above conflict with a preordained "right" or "wrong" course of action for Lucann or Thurgon? For instance, would it have made play more enjoyable for any involved party if I would have imposed the correct choice of:
Again, we are back to "preordination" being the only way alignment can be played. I think both characters are faced with situations where their beliefs are tested. I would not expect the factors you cite to dictate the only possible/acceptable game result. However, I would also find play pretty crappy if the answer is just "Well, once she was good, now she is evil. Kill her and take her stuff."
In my estimation, that level of GM imposition (by way of stick) contracts the potential future narrative and its emergent fallout.
To me, that is the level of GM imposition which alignment detractors seem to assume is the only way alignment can exist in the game, and is not supported by anyone as making for a good game anywhere in the hundreds of posts on this thread.
I definitely agree with this. That's why I'm puzzled by the frequent reiterations that "alignment is not a straitjacket". Whatever it is or isn't, if it doesn't affect player or GM behaviour than what is it for?
Again, you seem incapable of contemplating that something could, in fact, affect behaviour without dictating it 100%. In Manbearcat's examples of what could have dictated the characters' decisions, do you think it would be better play if those beliefs were simply ignored? Hey, we're back to my Devoted Servant of the Raven Queen animating an undead horde to advance her interests. No one can point to any indication that she dislikes Undead - I get to decide whether my character gives a crap about that, while also getting to decide he remains a Devoted Servant of the Raven Queen, even her Chosen One here on earth, and anyone who says different simply fails to grasp her will!