What do you think? Does being neutral mean partaking of either extreme in relatively equal measures, or does it mean avoiding them?
It means avoiding them. True Neutral is best seen as the philosophy of moderation in all things. The True Neutral believes that vice is extremism - there is nothing evil save that excess makes it so. Certain Greco-Roman, Chinese and Hindu real-world philosophies can be considered exemplars of True Neutrality.
In my games, neutralities are described by what they are passive towards or by what they don't believe is real. True Neutral doesn't believe either morality or ethics are real, not in the same sense that Neutral Evil does, because they also don't believe Evil is real - just an excess of participation in destruction. The True Neutral believes the world is balanced or would be balanced if people weren't all the time unbalancing it. It doesn't believe in "good karma" or "bad karma" - there is just karma. Participation in acts of extreme good or extreme evil are just as much vices as the other and so incur undesirable "karma". Excess of generosity is just as bad for everyone involved as an excess of cruelty. Wildly doing one and then the other doesn't bring balance, just more karma.
Followers of True Neutrality are very fond of the animal analogy to illustrate their beliefs - the spider eating the fly is good from the perspective of the spider but bad from the perspective of the fly, the tiger eating the deer is good from the perspective of the tiger but bad from the perspective of the deer.
The vast majority of True Neutral people would just see a person vacillating from one extreme to the other as insane. They would see no virtue in his actions because he's continually partaking in the vice of extremism and none of the wisdom of moderation. They would however understand the motive of a true neutral paragon who - in order to prevent an extremist from winning and unbalancing everything - worked to undermine the success of that extremist. For this reason, true neutral heroes could participate in a quest with a party that had an extremist agenda most likely if the party was on the side that would be perceived as the "underdog", because the outcome of that quest would presumably restore balance. But again, true wisdom here would consist of doing the minimum amount necessary to balance the scales.
It would be difficult for me to imagine a situation where a character wildly vacillates between extreme behavior and yet maintains balance sufficient for me to believe they are still in the middle of the map. For example, if a character is doing acts of extreme evil and extreme good for purely self-interested reasons, that suggests a strongly chaotic character and very likely a chaotic evil one. Conversely if the character was doing both for self-less reasons, that suggests a strongly lawful character. To get balance, the character would have to be doing both for disinterested reasons, which in practice I've never seen happen. Likewise, a character that vacillates between strongly serving the group and strongly serving the individual likely has some motive and is not disinterested. For example, their underlying motive could be "my group vs. all others" or benevolence or whatever.
Indifference to alignment ("unaligned") is the low intelligence (or at least low intellectuality) version of true neutrality. Every alignment has both an intellectual description made by people who have to systematize their beliefs and describe them, and a version that is not systematic and is acted on without a lot of cognition.
Social scores interact with alignment as follows:
a) Intelligence: Higher intelligence trends toward how philosophical and thought out your belief system is. How much can you justify your own actions within some sort of intellectual framework and how much do you feel the need to do so?
b) Wisdom: How much do you understand your own belief system and are aware of their own motives and actions? Higher wisdom trends toward better being able to label your own alignment. All low wisdom characters tend to perceive themselves as "Good" or at least whatever dominate social alignment is, regardless of their own behavior. High wisdom characters tend to "do the right thing" from the perspective of their held alignment without necessarily being able to explain why it was the right thing to do. Low wisdom characters not only aren't aware of their own alignment, they aren't aware when their actions depart from their own beliefs. They tend to make more mistakes.
c) Charisma: How persuasive you are in communicating your own beliefs and philosophies. The more charismatic someone is, the more that person is perceived as "a good guy" regardless of their variance from the alignment system of the viewer. Common examples occur in media of characters that are objectively jerks and evil but perceived by most of the audience as being "a good guy" because they are presented as charming. This can lead to fridge logic when the underlying wrongness of their actions is pointed out.
In my D&D games, True Neutrality is the alignment of the majority of humanity - the default bias that most humans have - and the remaining third or so of humanity is more or less equally distributed among the other eight alignments. That means only about 1 in 8 humans is good aligned, although because of the interaction of low Wisdom described above, usually more than half of humans believe that that are good aligned just because "Good" sounds better and is more openly trumpeted as virtuous.
