Okay you cleared up a lot. Technically it may be your injection pump. Since all your hard lines dribble. Plus you have cleaned out any crud that may have been present or not in the delivery valve.

I suspect the number one injection pump element may be damaged. To the extent it will still pump but is no longer able to reach the injectors pop off pressure required. The piston fit to bore inside the element is leaking more than is permissible.

If injection pumps are easily and cheaply available in your area I might locate one. If not cheap or readily available I would take a spare hard line. Cut it into two pieces and plumb a pressure gauge in. Then hang an injector off the end.

Then I would see what pressure is obtained when hand rotating the engine over in comparison to another element. One that you know is still hitting a release pressure.

Sorry but WVO type fueled engines are more prone to injection pump issues if that is the fuel you have been using. You also might have other elements in that injection pump that have more wear than desirable. Perhaps even being close to compounding the issue.

I did weight the possibility of accumulated fats in the base of the injection pump causing an obstruction. But feel since you see a dribble at each injector nut this is not the issue. I can of course be wrong.

It is important to mention what fuel was used early in a thread if not diesel number 1 or 2.. Not because people will pick on it as once was the case. Just changes the more likely possibilities. I am not sure either but with no mechanical lift pump this means no check valves that with an electric pump normally would be no issue. Yet still might account for your fuel back draining from the element you noticed. I was wondering about that earlier.