The stock PBG ST style pickups use a ceramic bar magnet on the bottom rather than individually magnetised polepieces, a plastic cover, a bit of wax, a bit of tape, and that's about it:
The bar magnet could contribute to the 90 edge rotation hum cancellation. I suspect separate individual magnetised polepieces could help but single coils will always have issues in noisy environments. Do the 2 and 4 positions cancel most of the hum? What happens if you sit a couple of layers of aluminium foil on the strings above the pickups, does that silence it?
Shielding is always the first step, and not just continuity of shielding but also thickness. Sometimes thicker shielding can be required to stop all the noise, like the thick under pickguard shield plates you can get. Also, whenever possible, try to have only one path to ground. e.g. if your shielding is grounding the pots then don't link the back of the pots together, instead just have a single ground link to one pot and have everything else link to that point etc.
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