Peace of mind does help any of us. It concievably might loosen up by use if it were a sticking element piston when hot. Depends what is exactly wrong in there.

Open hard line test is not that conclusive.

Only if the injector is not producing a spray when the engine and injection pump is hot. I could see any injection pump shop missing something like this. In many ways it is better than facing a head job at least.

If the cost is gettting out of control. You could get a pump lock and pull the pump and take it back to the shop if you prove this is still an injection pump issue first.