We use the Breastflow, and I always recommend it when people are having issues with pumping/bottlefeeding/nipple confusion, etc.
I couldn't find the link about the suck training you are supposed to do when using the nipple shields. This should make an easier transition when you wean from them. I think where I saw it was in Dr. Sears BFing Book. I'll look around some more. Also, my friend who used them said the LC instructed her to do it (basically you let the baby suck your finger) before using the shields each time. She didn't do it. Not saying that is the reason, but she eventually wound up EPing.
I was asking about the engorgement because it could have been a sign that he was removing less milk from your breasts. In your case (with oversupply) that might be a good thing, but just keep in mind you will need to nurse more frequently to bring it back up (during a growth spurt, for instance).