Suggestion: directory-up toolbar button.
Thanks for providing us with Kid3. It's been my default application to double-click audio files for years and it's fantastic that each new release brings new powers to make our lives easier.
Here's an idea for you to consider: it could be convenient to have an additional toolbar button to navigate "One directory up". I find faster a single click near the upper-left corner than moving to the lower pane, tell the two dots from the dot and click twice. That arrow-up icon could be located between the navigation arrows.
Since we can already enter a directory by double-clicking it in the files pane, navigation would not be tied to a static directory pane, so the lower pane might be reworked to include additional tabs, if that's possible. Those could show other information views such as bigger artwork, listings of arbitrary frames from all files (a reduced excel-like view for some fields may be handy to pinpoint errors without checking out each file), additional technical info of the selected file, a selection of User Actions, the tags of the playing file, global stats (sometimes I miss "Total duration"), favorite directories... There are lots of possibilities.
Maybe some users would not like to navigate with the toolbar button or lose room in the lower pane because of the tabs, and I wonder whether the main settings could have an option "Static/Tabbed" for them. Personally, without tabs I'd just resize-hide it if there's an Up button.