I've thought exactly the same as you that a helical spin would explain this phenomena. Now if the spins were randomly gaussian distributed you would see constructive and destructive interference but the net result would be zero (which is what the waveform summation is). Having bother positive helical and negative helical spins would explain the double slit experiment phenomena. Observation may remove the helicality (X/Y components but not Z) but the spin is maintained. Since the quantum particle is ballistic it would still pass through the slots or bounce back and show 2 amplitude peaks without the interference pattern. ie particle mode vs wave mode.