It sounds like most of the usual items have been changed, I would first of all check for fault codes to see if any codes have been stored which may indicate a problem with a component.
Usually this kind of fault is down to low pressure in the fuel rail which will prevent the engine from firing up - this can be confirmed with a diagnostic too reading the pressure sensor while the engine is cranked over - if it is not reaching the required pressure set by the ECU then the engine will not fire up.
All the low pressure lines should be inspected - you have said the O rings have been replaced but have all the seals been replaced on all hoses? If not then I would ensure all are replaced.
The fuel filter can cause problems & we have many problems with non genuine filters installed, so would ensure a brand new Mercedes Benz fuel filter is installed.
Injectors are another problem - were new injectors installed or used ones?
There are also seals on the fuel regulator valve at the end of the fuel rail - if this has not been inspected then I would replace the seals on this.
If all of the above checks out ok then the next step would be to have the pressure from the high pressure fuel pump checked - Mercedes has a special tool to connect up to check this.
There are of course many other issues that can cause the problem but would have all of the above checked first.
Let me know how you get on.
Best regards, Paul