I did try playing with setting expression and Vel X-fade to the same CC, and don’t remember thinking that it worked, but will try again adding also the dynamic range into the equation.
I doubt that VSL would go back and change levels on patches, but continue to think that some behind the scene correction might be possible. Like the "button" mentioned above, maybe an "automatic" (that’s the tricky part) gain adapter with a fader to either let different levels pass (like when you want a sfz), and when you want smooth levels.
I don’t understand anything about phasing and related subjects, but also think that the future for these VSL libraries, would be a VIP (3.0?) that would go beyond simple patch playing, but rather morphing (whatever that means) between various base patches to achieve the sound (definable though automation). I’m thinking along the lines of some advances of the libraries that I mentioned earlier.