Hi Steve
I can imagine you are tearing your hair out after that saga ! You seem to have checked out most things.
I can only offer "unofficial" advice pieced together from various sources. It's given in good faith but no guarantees ! My data is for the X250 model. I'm assuming yours is an X290 with a 2.3 Litre Euro 5b engine, which is likely to be similar but be aware that it may have some differences
Sounds like you have things mechanically OK and have good compression.
Slow cranking can be Low battery, Faulty Starter, Bad connection in the +ve feed cable or Bad connection in the Earth return. The last includes the notorious corroded engine to body earth strap, which can affect anything that earths to the engine rather than the body. This includes Starter, Alternator and Glow Plugs. Unless it has recently been changed, I recommend fitting a new one. Don't just clean up the ends of the old one, the corrosion is hidden inside the crimps.
The low pressure pump is capped at about 6 Bar. Once the engine is turning, the mechanical HP pump raises this to as much as 1600 Bar rail pressure. Normally it doesn't have to build up from 6 Bar as there is a non-return valve, but it can if necessary. Unless you get to at least 200 to 300 Bar the ECU will prevent the engine starting. If you are cranking OK but only seeing a very low pressure it suggests a sizeable leak in the high pressure system. I can think of 4 possibilities:
External leak - this would be obvious, so it's not that
Internally Leaking Injectors, but you have tried others and the ECU probably doesn't trigger them anyway if rail pressure is too low.
Internally leaking High Pressure Regulator Valve (Part of the HP pump), unlikely as this was renewed along with the new pump, but cannot be discounted.
No electrical drive to High Pressure Regulator Valve, so permanently open
The regulator valve N193 is driven by the ECU M010 by a pulse width modulated signal at about 1 kHz, so it can be smoothly controlled over a range of positions to control the pressure. There are 2 wires:
Power from ECU Connector A, Pin 19
Control from ECU Connector A, Pin 49
I think the next step would be to use an oscilloscope to check that N193 is getting the correct drive and is drawing the right current. You could also possibly divert its fuel return pipe and visually check that you don't have fuel returning during cranking, when it should be closed or almost closed.
For completeness I should mention that the ECU drives current to the Regulator Valve, The Injectors and a number of other actuators. This all takes power, so if there is any shortcoming in the power going TO the ECU it can affect its ability to deliver. Check out the various fuses supplying the ECU, they are listed on Elearn which I'm sure you have a copy of.
I wouldn't worry about glow plugs at this stage, as they are mainly needed for starting below about 5 degrees. They should measure about 1 ohm each when cold. Their current is monitored by the "Relay" and if not high enough it will throw an error. However, this could be the earth strap or a poor connection to the glow plug, and not necessarily a failed plug.
White smoke is unburnt Diesel, which is mostly seen just after a cold start (especially if glow plug(s) aren't working). If you see it any other time, it's a sign of massive over-fuelling.
Good Luck !