Fin, I would suggest that you go ahead and add clear line (sorry Ed Anderson)  between the fuel pump and the tank. If the bubbles are coming from above than you do have a leak above the pump.  Are the bubbles that you are seeing downstream of the pump being washed down with the flow ,or are they bucking the flow and coming back to the pump. If they are going with the flow , I think you can rule out cavitation and vaporization as possibilities. You have a positive head on the pump and it is at cockpit temperature is why I think this. I still hold out that you have an airleak between the tank and the pump since you say the vent is open.

Bernie Kerr