Worf insists that the truth about the cloned Kahless be revealed to the people at the end of “Rightful Heir”, so I assume that info was part of the press release when Kahless was crowned.

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”.