You bring up a good point about how Seven's Borg implants may have actually helped her to adjust more rapidly than someone who had been similarly traumatized and isolated for an equivalent amount of time. That said, it might be a wash, since her implants also harmed her in various ways: Seven's implants compelled her to revisit the site of her assimilation ("The Raven"), resurface personalities of assimilated people ("Infinite Regress", albeit they were malfunctioning), develop paranoid delusions ("The Voyager Conspiracy", though arguably this was again a malfunction). Seven's implants were also said to have inhibited her ability to experience the full range of emotions ("Human Error"), and that was by design. Still, I think your basic point holds, that it seems like Seven could learn and develop more easily than an unmodified 30 yo human.

But the question of Borg tech raises another issue that makes one wonder whether this adaptability could be attributed to her implants: What exactly were the Borg "maturing" in that chamber for 5 years? Presumably, they didn't stick her in that chamber for her personal development---the Borg would have selectively "matured" the parts of her body (and brain) that would enable her to function as a drone in the collective, nothing more. So it may be possible that the maturation chamber effectively "walled off" or "isolated" the parts of Seven's mind that would have been developing during that time to stop them from interfering with whatever parts the Borg did want to develop. As a result, even if the rest of her body was effectively 30 yo, her mind might still be effectively 6 yo (or maybe a bit older, depending on how the chamber worked).

In the case I just described, Seven's improved "adaptability" would arise more from her humanity---aided, ironically, by the Borg having preserved it during their "maturation" process. In a sense, Seven would not be a "feral" child who was left with the wolves at 6 and then developed from there. Instead, she would be a child of 6 who was left immune to many of the effects that her subsequent experiences would have had.

a waste of good genetic material

It's worth comparing this perspective to Spock's view in TWoK that Kirk not fulfilling his "first, best destiny" as a starship captain is also a "waste of material". In other words, Vulcans place value on a person expressing their truest and best self. That would jibe with the idea that Vulcan society would not place artificial barriers to people expressing their gender and sexual identity, since doing so would be viewed as a similar "waste of material".

In fact, the script specified that it be pronounced "KAY-nar": https://www.st-minutiae.com/resources/scripts/186.txt So that must have been forgotten/changed at some point, or maybe it is the auditory equivalent of "whisky" vs. "whiskey".

Minor quibble (but then what other kind of quibble would it be):

In DS9: “The Adversary” the leader of the Breen Confederacy in the 24th Century is known as the “Autarch”.

Actually, "autarch" was the title of the Tzenkethi political leader, not that of the Breen. To my knowledge, the title of the head of state of the Breen Confederacy was never spoken (except perhaps in the form of Breen static).

Loved it.

Some amusing details:

• For Pike's singing voice, he adopts a kind of Meatloaf/Russell-Crowe-in-Les-Miz style that is exactly the right mixture of masculine and adorable.
• The build up to the Klingon Boy Band: We know that Klingons love opera, heightened emotions, spontaneous group singing, and choreography (if you're willing to consider martial arts a form of choreography). La'an even explicitly mentions singing old sea shanties which would seem to be an obvious way to translate the Klingons into musical form. So naturally, I was shocked that the Klingons would not immediately assimilate into their new musical reality. I even told my husband, "I can't believe the Klingons would want this to stop!" And when it hit, everything made perfect sense.

David wasn’t aware that Kirk was his father

Technically, David did know that his father was an "overgrown boy scout" named Jim Kirk, but they had never met before and evidently knew little about one another aside from their mutual existence.

I take your point about wanting a more cohesive narrative, but I think there is a more important function served by the idea of shifting timelines. By allowing for the same essential historical events to occur just in different years, Star Trek can preserve what I think is one of its essential conceits: that it depicts our future.

I think the world of Star Trek was and is meant to be understood as a view into how we could develop, as a goal that we could achieve. Certainly, as a kid, that was why I found it so compelling. It showed me the great things that humanity could achieve if we decided to listen to and trust one another. It showed (admittedly not always very well!) that everyone has a place in the future, even people who are might currently feel hopeless, left out, or oppressed. While I can only speak for myself, I never felt that sense of purpose from other major sci-fi or fantasy stories. I may enjoy Star Wars or LotR, for example, but they don't mean as much to me because I don't feel like I or the humanity I know have a place in those worlds. They depict the dead past of a distant place rather than a living future that we could all have a hand in shaping.

I say that knowing that Star Trek is essentially fantasy, of course. My point is that, my maintaining the illusion that we are living in Star Trek's actual past, it makes us feel connected and invested in a way that is different from how we might connect with other stories. I don't know if that's the reason for introducing the concept of shifting timelines, but I think it still makes it worthwhile just the same.