And that would be in system settings/audio as well?
If a fader is provided by the HiFiBerry driver, it should appear on that menu, yes.
I´m in expert mode now, but there´s no fader there. Does it make a difference that I´m using my pc to access the configuration via x.x.x.x:8080?
I think the GUI could look different, if you access it directly. Check it, please.
I think the routing is done after selecting that output. There should be a fader somewhere to increase the input volume. If you can't find it, switch GUI settings to expert mode.
The output is as expected. Select the entry under the selected one, ending with "S/PDIF". I bet that's the mixing one.
No, mixing the card-input into the card-output happens inside the card, not inside RPi.
Go to LE's audio settings and post the list of available output devices.
In theory, the RPi can stream content (non-DRM) to a streaming client like VDR. In practice, it's too difficult or impossible (no VDR app).
That card uses I2S, and mixing is 100% a HiFiBerry driver question. The HiFiBerry driver should provide an audio device with mixing capability. If you can't find it on the list of LE audio output devices, ask the HiFiBerry support.
If you attach the microSD to the PC, kodi.log is at /STORAGE/.kodi/temp. Check that folder, it must have some content.
Did you used our search function? Results for "Wireguard": Click!
I'm not angry. As a moderator I know that very few users connect a cam. I just gave you a good hint to solve the problem.
Then keep the LED script (will trigger a transistor) and build this little circuit: Click!
Use pin #02 (5V) and pin #14 (Ground). No script needed (I hope).
Yeah, this is the wrong spot to ask for working cams. 
Better switch to a Raspbian forum.
Have a look at my instructions (post #9). I'm using an LED, which indicates power-on. That could be your fan.
Just in case, here are my instructions: Click!
Currently dev's are pretty busy to build the next stable release, so I think you don't have to wait very long.
CodeGPIO.wait_for_edge(3, GPIO.FALLING) subprocess.call(['shutdown', '-h', 'now'], shell=False)
Surround it by try / except:
try:
GPIO.wait_for_edge(3, GPIO.FALLING)
subprocess.call(['shutdown', '-h', 'now'], shell=False)
except:
pass1: Switch de-interlacing off. Read here how to do it.
2 (optional): If suggestion 1 fails, try the other settings described in that thread. You should set the GUI settings to expert mode.