@Cedric > Thanks for your feedback.
Unfortunately, the current build of rekordbox does not allow for viewing of nested playlist contents within the parent folder. Our teams are aware of this request and are working to implement it in a future release.
I haven't tested the MIDI signals for the DDJ-400 myself, but you should be able to make those changes using the MIDI editor.
As for the layout, you're asking for a Traktor-like display, which unfortunately, we have had very little demand for, but we'll add it to the suggestions list.