You describe what some people would describe as a type of living nightmare - these things can be extremely frustrating but it is very nice to read that there seems to be no acrimony between the different people affected.
I think in the end I would be inclined to work back from where the symptoms appear to emanate from (your ceiling or further back if identifiable). You don't mention the materials involved in the floor/ceiling areas or whether there is a "utilities space" through which all pipework passes - these questions are important because it looks like you are going to have to work your way back from your ceiling upward (possibly sideways too at times) until you reach the location on the pipes where the leak originates.
The fact that this seems to occur regularly for a two hour period each working day does suggest it is a sewer/drain pipe that is at fault (a joint ?) and, moreover, that logging the habits of people above you might provide a clue - does the water have any waste characteristics ? If it were a water supply/distribution pipe then the supply pressure would have to fluctuate substantially to produce an intermittent leak (in my view not likely) - a way to check that would be to install a pressure gauge in each suspect flat and monitor them at hourly intervals. Finally, what could be the culprit is some sort of overflow or relief pipe or a joint on it - these should be flushed to see what happens and ideally visually inspected along their full length. It is worth remembering that there can be a time lag between the water escaping and eventually showing where it becomes evident.
A heating circuit leak is also a possibility and the best way to check that is a bit complex but shouldn't be beyond any competent plumber - I regard this as the least likely source, but I wouldn't dismiss it entirely.