Should the underlay at the ridge be open or closed with full rafter insulation?

Isn't it actually not about moisture at all?

As I understand it, a USB is intended to keep rain and snow away from the insulation layer so that it does not become saturated.

So what exactly is to be done? Close the USB at the ridge or not? What exactly should be dismantled, remove the DIN covering at the ridge again and lay the closed USB under the counter battens?

In this case, it is also about ensuring that connecting the USB does not obstruct the ventilation. Ultimately, a system change for ventilation cannot be avoided. Rain and snow are not affected by this; both systems work in those conditions.

In this case, it is also about ensuring that the ventilation is not obstructed by connecting the USB.
Ultimately, a system change for the ventilation cannot be avoided. Rain and snow are not affected by this, as both systems work.

What exactly do you understand by system change and what needs to be done?

As it is now, it corresponds to a completely normal additional interior ventilation, with functioning rear ventilation. See your pictures. The system with additional interior ventilation is no longer desired, so a system without it. Post a picture of such a system and you will see the difference and also what needs to be done. --- I will not provide a solution here for liability reasons.