Interesting analysis.
I admit there are several balancing issues here that I might have to look at, and in the upcoming slave update (0.8-branch) there is a new variable that fits somewhat into this context – a value that stacks depending on how you've intimidated/threatened a servant in various means. This value will work to your advantage while persuading, even if a slave doesn't like what you're suggesting.
There are prostitutes in both taverns that'll give you sexual gratification (and more) for a pittance. Check out Furry's tavern and Watery Eyes (and soon Dockgrave tavern), and then go to "Private session".
I'd love to analyze your situation more closely, though. Were you trying to persuade the tribalistic/feministic slaves (Rebecca, Lovisa, Michelle, Juno), or the more docile ones (Amelia, Piper)? How much did you invest in Manipulate, Charm and Willpower? Were you trying to persuade a young slave with an older character?
Your suggestions are however interesting and, in a world like Raana, definitely realistic. I'll consider them more closely when I move on to the persuasion mechanics.