What I want to know is, what did the mob boss do with all that mermaid flesh? If he (or someone else) ate it, then the curse would pass on, same as with the cook.
I think the mob boss didn't know about the mermaid meat thing. There's a high chance Miyuki only told it to the cook, because even if she was so desperately wanted to die, she wouldn't want to "benefit" her torturer.
My take on this is the meat only passes the curse to one person, then it becomes normal meat, and eating probably speeds up the healing process. These speculations just make the story more depressing than I expected.
Considering that all above is true, why didn't Miyuki eat her torturer? The mob boss was totally bare in that torture chamber, and even if he had been amoured, she could still go after him times and times again till she succeed. This makes me think alot about Miyuki's attitude toward cannibalism.
Miyuki lost her will for living a long time ago, she felt pain but was numb to it, she gave no care whether she lived or died, that was part of why she didn't care to protest, to get out of her current situation. However shitty her life was, she never resorted to cannibalism though (if she had, the mob would have known and kept her in check), because "eating flesh" like that, in her mind, was the root of her misery. The "weird fish" she mentioned was clearly a mermaid, with upper human body. Her calling the mermaid a "weird fish" also shows that she still didn't want to admit being a cannibal, she was still conflicted over it, all the while knew exactly what she did, this would turn up later on.
At last, seeing the cook's human decency and tasting good food made with care probably reminded her that she was a human after all, not an emotionless monster. That was the point when she started to care for her life, and that was why she wanted to die, providing the cook with a mean to escape probably was a contributor, too. After realizing that, she finally refered to the mermaid as "she", I think that was the moment Miyuki owned up to her crime. The title "Primo Piatto" is translated to "first course of a meal", I know it's "the first of a three-course meal", but can we also interpret it as "the first dish"? As in, the first real food ever that Miyuki eat, which make her felt like a human and not like a cannibal - a thought that had haunted her ever since she ate that mermaid flesh out of hunger.
She passed her curse on to a person who was nice to her, yes, but it was the only way they both could escape. The ending makes this all the more depressing, seeing how the cook didn't condemn the crime of cannibalism, but kept falling into the abyss (those guys are assholes, but remember cook had to eat Miyuki first, and that horrible action probably pushed him over the limit). Well, at least we know Miyuki has finally let her guilt go and be free.
Tl; dr: Cannibalism is bad. Dowman Sayman is awesome.
I think the reason she didn't eat the mobsters is because she was mellowed out from the years. She did mention that she ate the 'fish' 300 years ago during a famine and was taken to the mobsters only 30 years ago. I bet she also had a strong cannibalistic tendency, especially right after eating the mermaid, but coupled with the famine and the long years, there's a high chance that she started to learn how to control herself to the point of normality. That's why she didn't bother to cannibalize the monsters, she already went through the difficult parts of the transition and was in an (unfortunately) relatively stable state
