Even if using optocouplers. I would wire the push buttons with twisted pair 
screen cable to the electronics box. Connect the screen at the electronics 
box only. This will help give you resistance against spikes getting into the 
cable, which will happen if it is bunched close to mains cable.

As an aside to this, do your wiring codes allow you to have low voltage 
cables in the same trunking? Most such arrangements require a minimum 
distance between mains and low voltage cables. 

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