assigne chain number to a modifier

Hello,

Quick question : each time I select a chain, whichever the device, I would like to store the chain number in a modifier.
See screenshot attached.

Exemple :

device 1 , chain 2 selected > modifier = 2
device 4 , chain 2 selected > modifier = 2
device 4 , chain 4 selected > modifier = 4

Is that possible using reactions ? I cannot find the adequate action.

Subsidiary question : is it possible to map selected chain's volume, mute and solo to just one single knob and 2 buttons ?

Thank you for your help !