Complex sentence: Her employers had unwisely used Mack as an ingenue (stock character) but RKO offered her a second chance as "Mamie Donahue" in "Sweepings".
Split into simpler sentences: Her employers had unwisely used Mack as an ingenue (stock character). RKO Radio Pictures Inc. offered her a second chance as "Mamie Donahue" in "Sweepings".