Solid questions, thanks for the thoughtful response. I can try to speculate. I should observe that I haven't studied developmental psychology or philosophy above an undergrad level, so I'm likely to misstep with both logic and terminology.
> the incentive to justify immoral behavior, and the lack of evidence that more tribalism would have anything to offer
First I can say that I don't think Kegan means that the relaxation of identification with abstract systems means devolving back into identification with tribe. I think it refers more to the ability to engage in good faith with criticisms of one's abstract systems, one's ideologies, and recognize their weaknesses and inconsistencies much as one did with one's tribe--that is, a transcendence of the need to associate weaknesses of the system with weaknesses of the self, with the associated defensiveness. I think anyone who uses those inconsistencies and weaknesses to justify immoral behavior probably didn't reach the abstract-system phase to begin with, though it doesn't seem unlikely that someone would feign that to display tribal identification. I'm guessing that's what people mean by the term "virtue signaling."
> the apparent correlation between a lack of faith in philosophical ideals and dishonesty, neoliberalism, lack of nuance, late capitalist culture, etc.?
I'm with you on this one. I don't think Kegan would advise sacrificing all faith in abstract systemic thought, and I don't think that's what Socrates is doing when, e.g., at the end of a long series of questions, he shrugs and tells a story he heard one time about the gods. I think what's at issue is whether one can separate oneself from one's ideals well enough to critique them, to balance conflicting ideas thoughtfully and not feel obliged to defend your system to the death when it butts up against other systems. I think what I called "lightness of spirit" above is key here, and we see it all the time in Socrates - the wisest in Athens not because he knew more, but because he knew that he didn't know more. I mean, so he says. There's obviously abundant lightness of spirit there too.
The charge of individualism is a good one. I hope that there's a stage past the militant adherence to first tribe and then ideology that doesn't necessarily entail individualism. Maybe it just looks like a relaxed broad view of the human tragicomedy that takes the good with the bad, sympathizes with the suffering, and makes choices according to conscience.
As for cartooning Socrates, I hope not. I'm certainly not caricaturing him mean-spiritedly. I admire his enterprise and methods, including when he has to set rationality aside and tell a good story. I think those moments make him more humane, more relatable, more realistic. I realize that my current representation of him is at odds with the point made in the article, but even here, choosing death according to conscience, it's still not about making some grotesque martyr of himself. I think it's admirable.