"Fliegendes Blatt" = (anonymous) broadside! Not any particular one.

"Lebenslust. Sechs zweistimmige Kinderlieder mit Pianofortebegleitung" (1893) is an exact translation of "Levenslust. 6 tweest. kinderliederen met pianobegeleiding" previously published, only in Dutch, by the composer's wealthy father (according to the footnote in the reproduction linked by Monique). Breitkopf & Härtel is an international publisher of high prestige, quite an honour! Some of the lyrics had to be freshly translated, whereas for Engelenwacht/Kinderwacht, the original German text needed very little tweaking - I have not yet seen the result.

Given the many printed books mentioning Güll as the author, apparently undisputed, the pre-Internet attribution to Diepenbrock can safely be called erroneous nowadays.

I think we esentially solved the puzzle, with united forces.