how about making the logic of the arduino so that in case it fails, output is LOW, then all the relays are DEACTIVATED and this closes the 220Vac circuit so all light will turn on.
I believe it is safer to fail with light so you can continue with your normal life, than all in darkness.
Also, you could prepare a manual switch board that could replace the arduino board by unpluggin the arduino and plugging this board and then you have manual control of each light. of course, a good recomendation would also be as you say, to have an identicall arduino ready in case the one working fails.