Anything with common rail injection, yes it takes a while to access the cylinder head/injectors. 4 hours is about right. Basically just like a diesel truck. You start removing stuff until you can SEE the engine, and then take more stuff off until you can GET TO it.
I suspect they're going to do a compression test because that's one of the "steps" in isolating failing injectors (or what I call in-JUNK-tors). Seemed to see a few of those this year but on different equipment/engines. Remember, with common rail injection, if you starve it for fuel, if it gets ANY water in the fuel, or ANY gasoline (basically anything BUT pure diesel fuel), it's hard on the supply pump, injectors, fuel rail sensors, limiter valves, etc. That is why I can't stress enough the importance of clean, fresh fuel--and zero water content. Diagnosis on these is more of a process of elimination than it is finding the failed part and replacing/repairing it. That's what I absolutely HATE about common rail, and is exactly why I'm considering changing professions.