Thanks for all the great details.It's sufficient, but not necessary, that the top of the coil is a noise receiver as Bill Lawrence indicated. Because it's definitely not the only source of noise. Usually, in a grounded system, magnetic noise is more prominent and that is why Bill focused on that aspect. That is why the Micro Coil exists, it specifically addresses that by reducing the area of the magnetic field around the poles, and thus the hum pickup (from the physics of coils - the wider the diameter, the greater the sensitivity to magnetic fields).
A lesser known subtlety of shielding is that it is not necessary for the shield to completely surround a device, to have some effect. That is why, for example, you can have a shielded pick guard and no cavity shield and still have some noise rejection. Even the fact that the ground wires in a completely unshielded guitar offer some protection, because they form a capactive voltage divider with the active wires and interference source, that reduces the level. It works because the guitar ground is earth grounded at the amp. But it's not as effective as cavity shielding.
Different environments present different and mostly independent levels of electrostatic and magnetic hum. This muddies anecdotal experiences and comparisons, especially when the difference between E and M interference is not known or understood.
Now I know why Bill Lawrence went to the trouble to offer his Micro Coil design - he was intentionally reducing the area of the magnetic field around the poles.
I also now know that a guitar doesn't have to be comprehensively shielded to have some effect (against electrical field noise).
And that last paragraph really sums it all up well, especially WRT differing anecdotal experiences and comparisons. Now I know why I've been seeing all sorts of varying advice on what to do to get rid of 'noise'.