Thank you pianoleo for being so patient and showing this pickup bar solution.
I couldn’t wait to get home to go through the score again.
And by chance I even found another elegant solution, which gets the same perfect result:
In the score I recovered the extra bar first by deselecting „End voice“ in the previous bar (this is just because I had removed the rests beforehand).
If one puts in an extra bar with time signature (and key signature) one can just delete that extra bar from the system track - that is all; the time signature and the key signature stay in place.
If one just needs a cautionary key signature at the end of the flow one can hide the cautionary time signature obviously.
Please have a look: Dropbox - fantastic workaround.mov - Simplify your life