Household water manifold - A flow meter on each output. Advice appreciated.

There is one main problem with you approach: this sensors based on hall effect are not design to identify leaks as they only operate at a very specific starting pressure. Say for example you have 10PSI in your pipes, if you install it before a tap and open it will read the maximum flow of water in this point. But, if there is not enough pressure in the outgoing water you will read 0. Also a very small flow of water outgoing the pipe will not be detected, as it generates not enough negative pressure. Usually leaks are very small flow and continuous.

You can simulate this by blowing some air in the sensor. It only starts to read after some air pressure.

There are other ways to find leaks, depending on the environment. I believe there is some kind of paint you can add to the water point of entrance so you may discover the whereabouts of the defective pipe. Also check all the toilet seats, specially if you use hydra valves.