peter s,
You said; "But shouldn't the alignment lines be parallel to the grooves if they occur at null points? This makes the cantilever parallel to the grooves."
Yes. It's the grooves that are not parallel to each other at the two null points. As the stylus tracks past the outer null point, the stylus tracing angle turns too far inward. As the stylus approaches the inner null point the groove angle turns inward even faster (nearer record center) and catches up with the stylus tracing angle at the null. Overhang staggers the null points (a line cannot be drawn from the Spindle through both null points) to make this possible. As the effective length of the arm increases, the arc gets shallower and shallower and the overhang is decreased, bringing the arc closer to ninety degrees to a line drawn between Pivot and Spindle - until you have no arc, no overhang and a linear tracking arm.
You said; "But shouldn't the alignment lines be parallel to the grooves if they occur at null points? This makes the cantilever parallel to the grooves."
Yes. It's the grooves that are not parallel to each other at the two null points. As the stylus tracks past the outer null point, the stylus tracing angle turns too far inward. As the stylus approaches the inner null point the groove angle turns inward even faster (nearer record center) and catches up with the stylus tracing angle at the null. Overhang staggers the null points (a line cannot be drawn from the Spindle through both null points) to make this possible. As the effective length of the arm increases, the arc gets shallower and shallower and the overhang is decreased, bringing the arc closer to ninety degrees to a line drawn between Pivot and Spindle - until you have no arc, no overhang and a linear tracking arm.