I agree that introducing a pacifier can cause decreased milk supply.
Milk supply can decrease quickly if baby isn't nursing well. Babies have a natural urge to suck, and if baby is fulfilling this need elsewhere (a paci), it will have an effect on bf. Even if baby isn't actually feeding and is just comfort nursing, the stimulation will increase milk supply.
Having a preference for one breast could certainly result in a blocked duct in the less favoured side.
Having breasts that feel soft does not mean her supply is low. When your milk comes in, your breasts will often get engorged, but once baby starts feeding, your breasts learn how much milk to produce. It's all supply and demand. That's why the engorged feeling usually only lasts a few days. Your breasts will adjust to the feeding schedule and produce the appropriate amount of milk. As long as baby is having 6+ wet diapers a day, he's getting enough.
Try to hold off giving bottles or pacifiers until breast feeding is well established, usually by 6-8 weeks.