>Try exchanging the red/black pair with the white/green pair
I think he's done that.
Get a few crocodile clips - you'll save an awful lot of solder(ing) - and swap hot and ground wires of your single coils. You can also try connecting the neck pickup to the mid pickup lug and vice versa.
Coils are wound in different directions and magnetic polarity also varies, so there are few variables here.
Even if the pickups are in phase and have the same polarity, one might have hum coming through because
the single coil and each one of the humbucker coils are not matched pairs. Which they have to be to buck the hum.
Generally, if polarity mismatch is the problem, you can change the polarity of most humbuckers by flipping the magnet 180*, but the L-450 has got slug magnets, so unfortunately that's not an option.