1) when coming up with a total current measurement, do I include the
current flowing on the neutral line? Should I indeed measure any
current on that line at all - or should the current on the neutral be
equal to the sum of the currents on the other 3 lines?


The current in the neutral will NEVER be more than the highest current on one of the 3 lines. It will ALWAYS be less than the highest current on one of the three lines unless there is only current on one line. If the current in all 3 lines are balanced the current in the neutral should be ZERO. This would occur if all of the loads were three phase motors.

If this were a 2 phase panel, such as almost all residential panels are, it would have 2 hot lines and one neutral. In this case if the loads are balanced the current in the neutral will be zero. With unbalanced loads the current in the neutral is the difference between the 2 hot lines. Because the current in one is 180 degrees out of phase with the other the current from one cancels out the current from the other. Best practice is to try to keep the loads balanced as much as possible.

With a 3-phase/4-wire panel the current in each line is 120 degrees out of phase with each other so the math for calculating the current in the neutral when the loads are unbalanced becomes more complex.

The above is based on simple resistive and inductive loads back 50 years ago when I worked with this stuff every day. Modern day loads with lots of electronics and switching power supplies`make things even more complex.