Hello and welcome to Just Answer.
Does it start ok when warm ? Be advised that when the o rings leak that this will usually cause extended cranking when warm. As the oil thins down, the oil can bypass the orings more easily. The injector driver module (high voltage driver module for the injectors) has been known to cause problems with cold operation . You might be barking up the wrong tree here. There are also common wiring problems that can affect the operation of the injectors and the glow plugs. .
How many miles on your truck? If there are a lot of miles on it, servicing only one or two injectors is not a realistic approach.
Have you seen any loss of engine coolant?
There are several ways of pinpointing injector problems. The powertrain control module (PCM) has a BUZZ TEST that will actuate the injector solenoids to allow the technician to test the injectors with the engine stopped. This gives an audible indication of injector activation.
The key on, engine off and the engine running self tests will check the engine management system for components that are no longer operating within determined limits. Once the engine is FULLY warmed up, the CYLINDER BALANCE test can be triggered through the Ford scantool to pinpoint non-contributing cylinders.
It takes a lot of training and experience to be able to build up a reasonable level of proficiency in the diagnosis and repair of this engine . I'd suggest that you may want to try to find a good Powerstroke technician.