I now have found out how to move the fingerings of the left hand keyboard
staff below the staff. I use the reposition text plugin, but the trick is,
not to filter for text before using the plugin. If you filter for text, the
plugin performs its last action without openig its dialog. So, don't filter,
but just select the area, that you want the plugin to perform its action.
Doing so, the plugin opens the dialog, where you can tell the plugin what to
do, and in the advanced settings, which text element to move. You can even
specify wich font size of text to move. In other words, the plugin filters
for you.

How do I write a silent finger change. I
know, that I just can type both finger numbers, but by convention, there is
a bow between 

To make a silent fingering, use a hyphen, like 1-2.