thank you for the advice. In the meantime I had already solved the problems, but keep wondering how they were created in the first place.
The solution was moving "keys.dat" out of the way after stopping the client, restarting created a fresh one with the default bindings in place.
I suspect that something is created in the wrong order when the whole ".atrinik" directory isn't there already; the Debian/Ubuntu package most certainly doesn't mess with users' home directories, because traditionally every user has their own configuration, and won't even need one for Atrinik unless they actually play it, so this is left to the application. The installer also cannot tell who called it, because "root" privileges are needed for installation. So I suspect that "keys.dat" might be created before the whole path "~/.atrinik/3.0/settings" is there and silently fails. I haven't verified that yet.
Please consider to remap "[" and "]" for turning left and right in future versions, because it is internationally unavailable, almost all Germans use a QWERTZ-keyboard with "ö" and "ä" in that place, and have those characters on "AltGr-8" and "AltGr-9" making them unuseable due to the needed modifier "Right Alt Key".
I found that I had selected the option to allow widgets to reside off-screen, but unselecting did not bring the desired one back. I played in windowed mode with two KDE panels at the top and bottom edge, so the usable screen height was lower than the total of 768 pixels.
The widget reappeared when i decided to activate all widgets in the configuration file manually, setting the numbers to "1".
Some weird things happened, though: the widget with the "attack state" graphics once got a surrounding frame, and I've now got two square shaped windows like those fior chat and messages. What are they and how are they called in the configuration file?
Is it possibls to accidentally switch off widgets that don't have a closing button and are not mentioned in the configuration options (like "FPS")?
I'm not sure how all this happened, but I think it would be best to start the game again as another user with no configuration directories in place to investigate if there's a reproducible problem with creating a default configuration.