I'm not 100% sure, but I think your second question might be a phasing issue. Are the pickups stock (and stock wiring?)?
Perhaps in positions 2 and 4, the middle pickup is out of phase with the respective pickup its combined with. If they are all stock, chances are that someone who re-wired them didn't solder the hot to the switch and ground to ground, but vice-versa.
If the pickups are not stock, then maybe they are wound differently. For pickups to be hum-cancelling but in phase, the middle pickup needs to be reverse-wound and reverse polarity.
Anyway, something tells me that theirs is something wrong with your wiring, if the pickups are stock. If they are not stock it can be the wiring or the pickup themselves that need modifications.