You would have liked this Liù, I think: she had the steel-under-sweetness that you describe, and the sheer command of voice to make you believe everything she said. Her "Signore, ascolta" was breath-stopping. (I would have liked her suicide to be a little less stylized, but in general I've found stage direction at the Met to err on the side of the ornamental.) I find it interesting that the alternate portrayal of love that she offers to Turandot is still bound up with death and dying—though that's a connection so ancient, I can hardly argue; and self-sacrifice for love is almost as familiar a theme—but not with a loss of self beyond the physical fact of living. She doesn't stop being Liù because she loves Calàf. If anything, in the moment when she makes this rather damn scary gift to Turandot, she is most herself.

I would be very interested to see a production where Liù and Turandot are portrayed as being essentially the same age
Definitely. They would make much more effective foils for one another that way.

I wonder what the vocal requirements for the roles are; is there something in the range or tessitura that necessitates a heavier voice for Turandot, and thus an older sound? (I didn't immediately think so when I saw the opera. The woman I saw as Liù could certainly have sung Turandot.)