Short answers, pulled from the story.
Grove's Dictionary of Music states that a work becomes opera rather than Singspiel when music develops the dramatic denouement. This genre relies on spoken dialogue to carry the narrative forward while musical numbers appear as ensembles, songs, ballads, or arias interspersed within the speech.
The word Singspiel appeared in print during the early 17th century when performances grew profane enough for secular use. The earliest examples emerged from German miracle plays in the early modern era where dialogue was already interspersed with singing.
Johann Adam Hiller and C F Weiße created Der Teufel ist los oder Die verwandelten Weiber in 1766. Critics later called them the fathers of the German Singspiel because their collaboration launched a string of similar works that defined the genre's direction.
Wolfgang Amadeus Mozart composed Bastien und Bastienne in 1768 followed by Zaide in 1780. His final contribution was Die Zauberflöte which premiered on the 2nd of May 1791 while other works included Die Entführung aus dem Serail in 1782 and Der Schauspieldirektor in 1786.
Singspiele functioned as middle-to-lower class entertainment distinct from aristocratic genres like opera, ballet, and stage plays. Traveling troupes performed most Singspiele rather than established companies in metropolitan centers so performances could reach audiences outside major city hubs.