"Kwan's voice has more innocence than Yeung's"
Sensitivity =/= innocence. It is possible to be innocent precisely due to lack of sensitivity, by not even noticing things that the more sensitive have already noticed. This is the case with Yeung, who clearly never even bothered to understand what the song is supposed to be about and is merely transferring the lyrics straight off the page into the mic via her mouth. She is innocent(!) of the meaning of the song. That is not a compliment.
"Fan's who sounds like she retained it from childhood"
Which is why for a few years she specialized in children's songs:
from which she got her iconic concert-finisher:
"Vocal training and care should not try to change one's voice or 'improve' it, but to prevent damage to their current voice.
I disagree. Some of the ultimately best singers started off with a voice that was unable to fully express the emotions they had inside them. In such cases they benefited from the change to their voice that happened during their career. The most legendary example is:
who found her true voice by attending a concert of her own idol:
during which as a fan in the audience she cheered so loudly for the duration of the concert that the next day her voice had changed and she was finally able to sing the way she wanted to in her mind. Thus she credits her idol for magically gifting her true voice to her through his music. (Miraculous anecdotes such as these abounded during the Counterculture era.) Having once found her true voice, she thereafter never changed it again, which is what I support.
Of course I agree that if you are fortunate enough to start your career with a voice that already matches your inner personality, you should not try to expand it. But not every singer is so fortunate. If they were not allowed to find their true voice, they would have to adjust their songs to suit their starting voice, which would mean their songs become less sincere as well!
"the vibe felt more casual and day-like than the dance songs by Amuro and Kwok. The video also suits the song. D&D also feels more casual for dance music. Why is that?"
Actually, that one itself was Polygram's response to the following earlier collaboration by Capital:
which then incited Capital to counter-respond with its second one. Where does this earliest one (Kwok being one of the participants) fall along your casualness spectrum compared to the other two?