Good news, what keymap should I use?
None. All important buttons are pre-defined by CEC. If needed, you can add functions to color buttons, or change pre-defined buttons.
PS: CEC is made for your original TV remote control. Only IR codes, which are implemented by the TV, can pass-through by CEC to LE.
Skripo Yes, CEC is easy. It's activated by default on LE, and works as you said.
I've already problems with this line: Copy folder "shutdown_scripts" to /storage
How can I do this?
Attach the microSD to your PC and copy that folder into the /STORAGE partition. Then do the rest of the instructions on RPi by SSH.
I don't have that case, but I think your chances are pretty good.
Don't waste your time, there is no mysterious LE switch to route your input to output. It's all about the HiFiBerry driver.
What's your HDMI setup? (which TV / monitor / AVR model, HDMI cabling RPi -> TV or RPi -> AVR -> TV)
If you can't get analogue audio, chances are your volume is set to zero (check the bottom of guisettings.xml for volume size).
UHD is higher than your TV resolution. The RPi has to down-scale UHD to 1080p, which obviously doesn't work well. Because UHD is not made for your TV, I suggest to ignore this issue.
In Germany we used to say: You're riding a dead horse.
You system is packed with banned add-ons. No wonder that it freezes. Because we don't support banned add-ons, I have to close this.
OK, then Kodi doesn't start at all, which means it hasn't generated the folder and the kodi.log.
We are still stepping in the dark. Raspbian has a tool to update the RPi firmware. Maybe LE can boot with a new firmware.
OK, it was worth a try. It's the mix of GUI and video streaming that stresses the GPU (not the CPU).
Your idea of upgrading the CPU to get better LE speed doesn't work. It's as I said earlier: Don't invest into that old system.
You can try some settings from this thread. Especially switch off "sync playback to display" and "de-interlacing".
Is there a utility for this and an example for us that are not as savvy, please.
A starting point is our Wiki: Click!
Make sure you connect a digital source (S/PDIF). The card will not accept analogue input. Good night. 
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.
