It is good that working with fuel scares you. That way I hope you are careful and take the proper precautions.
The flow we are talking about is from the electic fuel pump at the rear of the car only. The fuel injection pump (FIP) uses a very small amount when car is running relative to the 1 liter in 15 seconds quantity of flow the system is capable of. The fuel injection pump does not "speed up the fuel", it sends a very small amount to the engine for combustion.
When the system is all hooked up, the flow is the same (almost the same) at the beginning as it is at the end of the return (same from the perspective of how we are measuring with a container and a watch). It is a solid mass of fluid, with very little compressibility, so what goes in must come out at the same "speed" or flow. By disconnecting the line in various locations, we narrow down where the restriction is.
The car does not need to be at more than an idle. In fact, to lower risks, I did my fuel flow test with the engine off (with a good battery).
The good new is you found your issue !!! 250 ml in 15 seconds is not enough and IMHO you will be in much better shape, or cured, once you find and resolve the restriction.
Now measure the flow at the inlet and outlet of the FIP. Others have reported issues with something at the outlet of the FIP (I believe a check valve).
Isolate the fuel restriction before you start taking things apart (like the damper or FIP that you mentioned).
I also believe that there is a small possibility that the fuel pump is bad even though it is putting out 1 liter in <15 seconds, since this is without any restriction. A fuel pressure test will answer that question. But, if it was me, I would leave that for last and see if a specific restriction can be found.
Good luck !! I know what it feels like to get past this one, and I know you will like it too. Enjoy the ride.