The resistor is again a bypass resistor (as far as I know) but 500k won’t bypass a lot, if any.
I'm sure you're correct, and I did read that somewhere.

I’d think you need to look more at the phase shift value of the capacitor rather than seeing it as a conventional high pass filter.
FWIW, all the live testing I did, I only used the 500k resistor, but changed cap values.

What’s the rest of the control arrangement like, standard LP type wiring? Had you tried a 1meg volume pot fir the neck pickup?
2 HB /1 v /1 t/ 3-way toggle