You seem to be on top of it. The plastic device in the vent line is I THINK the overfill limiting valve, so you might want to see if it's causing your fueling problems at the pump.
I am sure it is. Some one took out all the vent lines and the tube in the trunk of my 70 Dart before I got it. It still vents because of the venting cap but filling up the tank sure does suck. I put my ear down near the filler and listen for the sound of gas filling the feed tube.
