Yes – it certainly is a curly one, but I’ll have a go!
I doubt that either insurance policy would cover the damage, as the cause of the problem (the leak) would have been by definition a gradual occurrence, and not therefore a point-in-time “event” like a burst pipe.
The original construction of the floor in the upstairs bathroom and its underside being the original ceiling of the bathroom below, would be Common Property and as such the responsibility of the Owners Corporation to repair.
Repairs to the shower, and replacement tiles in the bathroom (below) would be the responsibility of the Lot Owner.
This is the black-and-white approach to the problem, and I agree that there are grey-areas – like a gradual leak causing the common floor/ceiling to suddenly collapse, and the latter being perhaps considered a defined event for insurance purposes. Have you asked the Insurance Companies for a ruling?