Dave
The current in the neutral will be 0 amps if the currents are equal and opposite polarity as is the case when both loads,line 1 and 2 hot to neutral,are exactly the same(resistive or capacitive/inductive or a like combination).
If the load on one side is reactive and the other side is resistive the currents do not cancel to zero in the neutral because the current leads the applied voltage by 90deg for a capacitive load and lags the applied voltage by 90deg for an inductive load.Fortunately not all loads are purely reactive but have some resistive component which does allow a portion of the current to balance out.
This also brings up the subject of harmonics created by some loads(computer switching power supplies)that do not get canceled in the neutral because there is no equal and opposite phase(180deg) current from the other side.
The answer to case 1&2 is the neutral current will not be zero for the reasons stated above.
Chris
