Jon,
In a single phase center tapped system, any multiple of 2nd harmonic current will be 'in phase' and add up on the neutral even when the fundamental currents balance out.
I don't think so. If they are fed from the two hots, the same order harmonic from one hot will always cancel out the that harmonic on from the other hot on a true single phase system. The two hots come from opposite ends of the transformer winding are "in effect" 180 degrees out of phase. All of the harmonics, no matter what order they are, will also be 180 degrees out and cancel in the neutral.
Don
