OK, so I managed the keyboard to fully work using the guidance from qobi in the first few pages of this thread.
Now I'm happy

Now the only thing which would offer more comfort is how to issue the setxkbmap automatically after the keyboard gets connected so that the pc105 layout will be automatically applied (otherwise I have to issue it every time I turn on the keyboard - although the keyboard connects automatically now, the layout is not remembered from the last session), and/or if there is any way how to eliminate the need of typing one character on the internal keyboard before the pc105 layout gets applied.

Anyway, I'm more than happy with the way it works now, if we managed to solve above mentioned issues, it would just be a nice bonus in addition to the working keyboard.

JoHnY