The wiring loom to the injectors can fail, easy to change. Could still be injectors, mail me I may have some. Would need to see if they are compatible.
Why would the wrong spec oil cause a misfire? Although PD’s do need the “proper” oil due to camshaft design with the Injectors. If the engine had been run enough to cause damage it wouldn’t get any better once the cam followers had worn out.
The PD engine isn’t a common rail injection system. It has unit injectors which are given a helping hand with pressure from the camshaft.