I have recently installed 2 PIV's ( Positive input ventilation ). One in a bungalow where the owners use their heating all the time and one in a rented a flat, where the tenant didn't use the heating very much, dried laundry on the radiators etc. Within 48 hrs the issue was cleared in both...
It's M.I.C.C Pyro to most. they are clamp boxes. It should have a fly lead coming off it to the accessory.
Cable should be fine, but as said before an E.I.C. report would be advisable.
Shared neutrals are dangerous. How would you certificate your work?? Put all the lighting on one circuit? They should be split before you attempt to change the board.