I’m going to dip my toe in the water here and say that the isolation valve is effectively common property (as it controls water that’s part of the system for the whole building) and so you should be telling the unit owners that they need to replace it. Alternatively, you could replace it at their expense.
Regarding the notice for turning off the water, there are no hard and fast rules but you could pass a regulation saying that it has to be 24 hours in advance and that it must stipulate between what times the water will be turned off.
Just as a genral note, the renovator has clearly done their job on the cheap, and now you and your neighbours are paying for it. Time to turn off that tap (ouch!). Oh, and put a lock on the building’s isolation valve so that this can’t happen again with anyone.