I am not a mechanic but...

250bar should be enough for any common rail injector to inject when about 70Vpp short pulse is applied to the injector solenoid. At the constant pressure, the quantity of fuel injected depends on the pulse width. To measure both the pulse amplitude and the width, an oscilloscope's needed.

In addition, it isn't important whether the ECM was told about exact injector specs or not. If not, the engine will work more roughly, with a bit less power, asking for more fuel but, that's all.

It is hard to say about the return fuel flow, I think it doesn't matter much, injectors can't stay new all the time.

Have you checked the vacuum leak at the injector's back leak off connectors? Their rubber o-rings tend to dry out with time. Also their plastic clips that should keep them tight to the injectors comes with pretty bad quality. If there is a vacuum leak at them, the common rail pump sucks the air at one or all those connectors. If need be, use plastic cable ties to tie the connectors to the injectors properly. If you have ever found some diesel on the rocker cover, around the injector seals, those connectors are the culprit.

Disconnect the return hose from the fuel filter and hook a vacuum pump to the hose. There shouldn't be a vacuum leak there at all.