Thank you!
It was a good idea flashing your kernel to confirm. We did this at the very beginning when the screen was blank but I forgot to repeat that on newer builds. Thank you.

Yeah, I have been thinking about to try the bluez5 backport but probably it will be easier to apply an already proven working solution.

Therefore what I will do then. I will scrap the current kernel, as maybe there could be left something that I forgot to revert, and download the whole again. (I'll keep my current defconfig of course.)
Then I will apply the change into the fs/proc/base.c and these commits, if they required, above.
Guess with this new solution, I don't need the bluetooth.service in the /lib/system that was suggested by the hadk.

Slowly we get there.

Edit: On your question to ssh, I cannot try that now as I am not at my PC. I will try and report back.

EDIT2: Oh, and forgot to tell that when I planned to apply the backport changes, of course I wanted to use 'droid-config-hammerhead-bluez5' rather than 'droid-config-fp2-siphon-bluez5' from the example. (But the backport idea is dropped anyway. )