So if one side says the war is over, even when it isn't, the Geneva Convention doesn't apply?

But surely the implication of that would be that, by failing to declare war in the first place, it will have been ensured that the Convention wouldn't apply anyway? All sounds a bit strange to me.

Not that it makes a great deal of difference in this case, where the major player appears to have few qualms about flouting both the Convention and the Charter.