there is no reason you couldn't have conflict between types of Good
From the point of view of mortal actors in the mortal world, I agree.
LG types believe that the only path to universal wellbeing is social order; CG types, on the other hand, think that social order is a burden on the individual self-realisation that is the only path to universal wellbeing. So they can come into conflict.
But once we move to a MotP/Planescape-style presentation of the outer planes I think it doesn't make sense any more. On this approach, the Seven Heavens is an exemplar of the truth that social order can produce universal wellbeing. At the same time, Olympus shows that the pursuit of individual self-realisation can produce universal wellbeing. So there is no reason, any more, for LG and CG types to disagree.
This is the problem with what (in the OP) I called "static" alignment: it tries to present all alignments as true - which means that it would be an error to deny the claims of any alignment - yet to present them as contradicting one another. Which makes no sense.
Whereas what I have called "dynamic" alignment - the idea that alignments are
convictions, whose truth will be tested through the actual play of the game - doesn't involve the same incoherence. Devils, for instance,
think that by spreading social order they increase their capacity to impose their yoke upon the world; but paladins disagree, believing that the spread of social order will pave the way for widespread wellbeing and flourishing. Both convictions can't be true - the play of the game will provide answers.
In my alignment interpretation, different alignments make things easier for their adherents sometimes, more difficult sometimes. None of them guarantee victory or defeat in arbitrary circumstances. Some may suit the themes of a particular campaign more than others. All serve purposes in the gameworld, and in the game. All contain some truth, none hold all the truth. All have virtues, all have flaws.
When you say "all have virtues" I assume you are referring to
virtues from the point of view of game play, not
virtues within the fiction. Within the fiction, I would have thought that, virtually by definition, there is no
virtue in evil.
The way I see it, using my "dynamic" framing for alignment, the interesting alignment dispute from the point of view of gameplay is not between good and evil. To try and make that clearer: in a game focused on good vs evil, alignment does not raise any particular issues for game play. The good guys know (by definition) that they are good, and similarly know that the bad guys are evil. The interesting aspects of play will probably not be moral histrionics but rather practical questions about whether or not the good guys have got the capabilities and resources to defeat the bad guys.
Within my "dynamic" framework, the obvious focus of alignment conflict is law vs chaos. That is, is social order an effective constraint on individual caprice and a basis for universal wellbeing (as the LG and CE believe), or is social order a burden on wellbeing because a basis for the brutal and powerful to impose their yoke upon the world (as LE and CG believe)? In a campaign that focuses on this conflict, victory isn't just about amassing and deploying the means for defeating evil, because it's not clear what means (law? or chaos?) are required. It's about engaging in, and making moral sense of, the different possibilities for human action opened up by different forms of social organisation.
I've GMed two campaigns that really focus on this question (or at least on ideas in its neighbourhood). One was an Oriental Adventures game (using Rolemaster as the system) where the "social order" at issue was the series of pacts reached between the Heavens and other immortal actors, which (in the view of the PCs) had hung a whole lot of mortals out to dry. There were also echoes of this concern around whether pacts uphold flourishing or rather serve as a yoke in the unfolding of the fedual political dramas in the mortal realm. In the end both wellbeing
and social order were restored, by the PCs forging a new set of pacts (both celestial and mortal) that were better accommodated to the demands of mortal existence. So in the end a victory for LG over CG, although at many times in the campaign thinks seemed to be moving the other way.
The other campaign with this sort of focus has been my 4e one - which, as it comes to its conclusion, turns out to pivot on the question of whether restoring the divine order of the Lattice of Heaven is or isn't the best way to deal with the threat posed by chaotic forces and the Dusk War that they seem to be threatening.
In thinking through the idea of True Neutrality, as per my OP, I have identified another (less obvious) possible focus of alignment conflict within this "dynamic" framing, namely, whether
all intentional activity (by humans and kindred beings) is a source of disruption and suffering. I don't think I've ever played in or run a game that made this question the focus of play, and am not 100% sure how it would be done - because FRPGing depends on players being active in the world (via their PCs), and so it's not clear to me how they might achieve, by their action, the outcome of showing that the solution is inaction (I am ruling out heavy-handed GMing used simply to prove the point). But I think it should be able to be done.