A performance can have up to 16 parts. Up to 8 parts can be controlled via the internal keyboard by turning on the corresponding keyboard control switch.
Question 1 :
Is the keyboard control switch only available as a single performance part parameter or can it also be changed by the currently selected scene ?
If it would be implemented as a scene parameter, I could control up to 16 parts of a performance from the keyboard by assigning these parts to different scenes.
Question 2 :
If I keep the keys pressed and select a new scene, will a part continue to sound until I release the keys, when it is not used in the new scene ?
With regards to your second question see this post
https://yamahasynth.com/index.php?option=com_easydiscuss&view=post&id=6369&Itemid=851#reply-6384
and as I understand the answer is NO. The sound will be stopped immediately.
If it would be implemented as a scene parameter, I could control up to 16 parts of a performance from the keyboard by assigning these parts to different scenes.
You seem to know that eight Parts can be under Keyboard Control, not sixteen, not even with your "new" math. It is NOT a Scene parameter, per se. Trust us, it's eight Parts under Keyboard a Control, Parts 1-8. Never 16.