After everything I have read here I would suggest a hardware problem.
(I am an electronics engineer though so there might be a bias there)
I would try adding a LowPass filter + extra pull-up
(10K resistor to 3.3v, and 33nF cap to GND) to the IO pin on the board,
and see if that helps.
If your cables to your wall switches are (too) long > 1 mtr. and not shielded they can pickup noise.
Try to add a resistor (4.7K - 10K) between 3.3V and the GPIO your switch is connected to.
I have implemented RC filter, and wires running from Sonoff 4CHpro and buttons are cat6 individually shielded pairs