One detail that I've noticed repeated in several of the links, and links of links, is the following claims:

• The first of Kegan’s adult stages ... the Socialized Mind. ... Fifty eight percent of adults operate at or below this level of mental complexity.
• The second adult stage Kegan terms the Self-Authoring Mind. ... Approximately 35% of the adult population is at this plateau of development.
• The last adult stage Kegan calls the Self-Transforming Mind. ... Less than 1% of adults attain this level of consciousness.

So where are the other 6+% of adults?

Also, you could have easily explained constructive development theory here. It doesn't seem particularly complicated. In fact, I put off reading this post for a while, waiting until I'd read the links, but then read it anyway, before I'd read more than a fragment of the first link. My understanding didn't change after reading any of the links, or the links of the links.

But I can see how this could be a very useful idea, even if I think statements like this one are basically bullshit:

Kegan’s and similar theories have been validated by researchers all over the world ...

Just based on what little I've read so far, I'd peg this as roughly as 'true' as the Internal Family Systems Model - Wikiwand, which I find immensely useful.