Could also be a drive pressure leak too, so between the heads and the turbo. Those are tough to find visually sometimes But will screech under acceleration.
You may now have a slight leak in the plumbing which is releasing the pressure, or the headgasket sealing area is now contracting enough when cold to release the pressure. Either way, a compression leak at the head gasket is a dynamic issue.
Some things that happen are that the injector hold down wears or bends slightly, the factory hold down bolts can stretch pretty easily, the threads in the head can wear, and the sealing surface of the head can wear.
If you want to be 100% sure it is the fuse panel and not wiring, unplug the fuse panel connectors and test for continuity at that relay socket. But I'm betting fuse panel.
The DEF injector, among many abnormal operating conditions, can mess up a sensor or cause readings to be out of range, but it won't kill the module communication