Update as promised:
Using the personality in the desk, and when viewing the DMX Output charts under the Z Key, the values corresponding to CMY remained on '0' even after changing colour parameters, or running colour effects etc. Definitely confirmed it was a personality error at this point.
So using the inbuilt fixture editor, I took a copy of the existing personality and amended the CMY control channels to be 'Other Colour' control parameters. This worked and enabled the encoders to control the CMY parameters, however colour palettes or picker didn't work as the desk didn't know what colour they actually were.
So I then edited the 'copied' personality, changed the 'Other Colour' parameters back to CMY, which then worked as it should have done so in the first place. I now have full control of CMY, using colour palettes or the colour picker!
No idea what was going on, but quite the oddity and an annoying time sponge trying to resolve!
Either related or unrelated, I also had the desk crash four times throughout plotting, so will forward on the crash reports to the team after the show is done.
Thanks again all for suggestions, research, and the created fixture profile @kgallen