Then the main stop cock would not have helped for some time, the entire hot water header tank in the loft would have to drain down through the hot cylinder before the leak stopped. What would have helped is the airing cupboard usually has a valve to stop the header tank supplying the hot cylinder. And you really ought to get your stop cock fixed. Or at least find where the one in the road is for your house, though that may also have seized.
Longer than the seals in the isolator valves. I generally find by the time anything fails the isolator valves are useless and either won't turn or spray water out of the gland.
Are you sure? I presume you have a combi boiler, hence the gas usage on-demand, so the water is at mains cold-water pressure, as opposed to the lower pressure you would get with a header tank system.
It could have used gas once the hot cylinder temperature dropped and needed to be brought back up to temperature. But that would have required the hot water to be timed to be on in the afternoon, which is often not the case with a hot cylinder since there is an assumption it will last until early evening.