To be clear, the one way valve on the fuel feed (starting as close to the pump as reasonable possible) is to help diagnose where air is leaking in. It's not intended as a permanent fix. It will give you one of three results:
1. If the leak is between the tandem pump and the one way valve then the car will start and very little air will be seen in the clear fuel return pipe. This is because the one way valve will prevent the head difference between the leak site and the level of fuel in the tank from sucking air in at the leak site.
2. If the leak is between the one way valve and the tank then the car will start and lots of air will be seen in the clear fuel return pipe (or the engine will splutter to a stop). There will be no air between the valve and the pump hence it starts, but then air will be drawn in through the leak site further upstream. This will be bled out by the pump and seen in the return, or it will stop the engine.
3. If there is still a very poor or no start then the leak is in the pump.
Given the history of the problem you have described, I would suggest that the original pump was probably not at fault and it was an air leak in the feed pipe all along. You have disturbed this air leak site in the fitting of the new pump making it worse. This gives a clue as to which fuel feed pipe to replace first. philward
suggested the feed pipe an unlikely source of the leak (while acknowledging it's an easy fix) but I have found the more they are pulled off and replaced, the more likely they are to leak. I once spent about a month (not full time) chasing around a slow starting issue. I clamped up that feed pipe nice and tight every time I refitted it but that's what it turned out to be. If the pipe "clunks" into position when you push it on you might think that's a nice fit, when it's actually indicating the rubber has gone hard and is no longer malleable enough to seal.
Answering some of your questions directly:
Your plastic tube clearly shows air and fuel coming out of the pump, therefore the feed pipe is the other one.
The lengths of the pipes are irrelevant provided they are air free.
You don't need a clear pipe on the fuel feed necessarily, just the return to see what air the pump is dealing with and when.
Any 8mm diameter diesel fuel pipe will do for testing and indeed replacement if necessary.
If the leak were at the filter housing it would likely leak diesel out given the filter housing is low down in relation to the pump level. This kind of (no diesel out) leak tends to be high up in elevation hence starting near the pump.
suggestion of brimming the tank and parking on a (steep) downhill, or car service ramps might well yield results in the form of a physical diesel leak. You would have to go round and carefully check every joint in the feed line (perhaps using kitchen paper) for signs of diesel damp.
I have had about 12 audi fuel delivery systems (not all A2's) in many pieces and adapted them to run vegetable oil over a period of about 20 years. The oil hardens the rubber hoses quicker than diesel and the extra viscosity of the oil creates a bigger vaccuum in the feed pipe than diesel (which is why i fit a lift pump at tank level). Thus I probably have more experience in this area than many. I apologise if this experience might be clouding my assistance.