I don’t think that’s true. At the start of the series, Mermista had lost all of her political power by being a bad ruler. So Adora helped Mermista get her subjects back, and then had access to Mermista’s subjects as allies. She could have just skipped the middleman and recruited the ocean people directly. Same goes for Dryl, they could have rescued/recruited the servants and then left. They didn’t need that purple haired abuser, she was a force for evil the whole time.