How much tank elevation are we talking about? Normally an open return line should not be bothered with 5 or 10 feet of lift. We normally route our fixed installation return lines up almost to the ceiling to get them out of the way and do not use any check valves.
Various fuel injectors will leak-off differing amounts of fuel - from almost nothing to a fairly steady drip. How much will depend, in part, on condition of the injector/s.
On your HR engine mounted filter, do you have the leak-off connected to a fitting on the top of the filter? If so, it might be a factor that the non-return valve portion of that fitting is faulty.
dieselgman