As I understand it, The operation of the fader in in the mixer should be reflected in the instrument volume in Kontakt.
The reason I ask, is that I have one library that won’t comply.


It’s not completely true. It depends, if you are operating by faders of the Instrument track, or a MIDI track. This is true for MIDI tracks only.

The “MIDI Faders” are sending MIDI CC7 to the Virtual Instrument. If the Virtual Instrument use this MIDI CC as a moving of faders (which is kind of standard), then it should works.