You said that you had checked the supply pressure being 4+ bar. So did you measure this or rely only on the diagnostic rail pressure value? Like you suggested the rail pressure sensor and circuit is very inaccurate on low pressures like under 10 bar. These have the old style inline fuel pump on the supply side but no monitoring for supply pressure like on the older M57 (DDE4). On those situations the vehicle doesn't start can you hear the inline fuel pump activating when you turn ignition to pos.2?

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey