It sounds like your tank is not venting properly. I'd replace that return line, and blow it back to the tank. I'd also try to pull fuel from the tank with a vacuum pump. I'd do it before removing the gas cap, and then after. It should pull the same in both instances if the tank is venting properly. I'm not sure how the venting works on MK3 cars, but there may be a gravity valve that's stuck.
Regarding the return line. The one on my Rabbit had been weeping for some time. My mileage sucks (like 36 MPG in the winter). The last 2 winters, I've had to replace the fuel filter on the highway after only 10,000 miles. They would freeze up and start sucking air. Both times, it was ***ty Exxon fuel that was the immediate culprit, but I could never explain how the pump could suck air from the tank. I'm starting to think that it was actually sucking air into the pump from the return hose.
Just yesterday, I tried to start my truck in 15F degrees weather. The truck has never had a problem starting in those temps, though it will chug a bit on startup. Well, it wouldn't start. So, I shot some starting fluid in there (BAAAD IDEA). The battery shorted out and did not crank at all. So, I had to pull the injectors to blow out the ether. Then I checked the plugs while they were out: all glowed great. When I had the injectors out, I pulled the OUT bolt off the injection pump. I noticed that the fuel in the pump looked low. So, I ended up putting about 1 cup of fuel in it to top it off. It's not leaking, so the only explanations are that it was not pulling fuel from the tank due to gelling (not cold enough for that, and I run an additive), or it was sucking air into the pump (likely thru the return hose). It also turns out that one of the return nipples on my #1 injector was broken and hanging on by a thread. I had to replace the body on that one injector.
After all that, and 3 glow cycles, it fired right up.
Bottom line: If you have air leaking in (usually indicated by fuel leaking out), fix that. But in your case, it sounds like you're not properly pulling fuel from the tank, either due to a blockage or improper venting.