Thank you for the quick answer.
Now this is strange … it works like you said, but I tried exactly this two times before plus one time by editing the inputconfig_p1.json directly. While doing that, I noticed that the game only saves newly bound keys, not cleared keys or the whole keyset. Thus, it can't "see" voluntarily unbound keys.
Like I wrote above, I even checked the saved alternative keys after setting them and they still got reset. After that, I resorted to clear them because its one keystroke less than re-binding them.
And now, it works …

I will observe it.
Thanks again for encouraging me to try this one more time. However, it
is a strange behaviour that a game will "fix" keys the user has deleted, forcing him or her to bind them to some junk keys. I've never experienced this with a game before. Why have the "clear" button at all if it only works until the next start?
Again, maybe this applies for a fix in D:OS 2.
Best regards
Raban