I haven't found a tap anywhere on the fuel line, but it's a bit hard to access parts of the fuel line (inside the frame) without raising the tip, which again is difficult when the engine won't start - so I can't bee 100% sure there isn't one. Anyway, my lift pump is mechanical and of
this type. I only have one fuel filter, and that's placed between the lift pump and the injector pump (that's the one I just changed). There is however a very "simple filter" inside the lift pump itself, which is just a brass mesh if I remember correctly. That one is probably instead of anything at the intake in the tank, which would make it possible for something to get sucked into the pipe in the tank and partially block it.
The slow return of the lift pump lever isn't a defect of the lift pump itself, since both the old and the new pump does this, and this is not the case the first couple of pumps. It's obvious that either a pressure builds up after the pump or a vacuum builds up before the pump causing the slow return. I guess the only way to find out where this is is to loosen connectors different places on the fuel line and see when or if the resistance goes away.
My worst fear is that something's gone wrong with the injector pump itself, and that it won't let the fuel pass freely thus building up pressure. I don't know about enough about the inner working of the injector pump, but maybe for example a stuck "engine stop" valve could cause this? At this point though, I'm still trying to find faults anywhere else but the injector pump...
I understood what you meant by the plastic hose, I would never get my head down to the pipe anyway. I'm worried about the seal between the plastic hose and the pipe, since the connector threads seems like something in the range of 3/8" and I don't have any clear hose larger than around 1/4" inside diameter.