Good examples of the -in in action. I've also seen Schubert's song cycle translated as "The Pretty Milleress". It's true, the titular maid of the mill is the unmarried daughter of the miller, but in this case the -in is just indicating that's she's female not specifically unmarried.
Like you showed with your examples, the -ess ending in English is a good parallel and gives that same kind of grammatical femininity as the German -in. As in, actress, heiress, temptress, mistress, hostess, and the ever-loved doctoress.