I'm very familiar with many variants of the selkie story, as well as the Melusine tales. The story for An Maighdean Mhara, according to Mary Mc Laughlin (who has worked on it pretty thoroughly as well as speaking the language) is, as I said in my post, a slightly different one. Heartbreaking, too. The Welsh mermaid (not a selkie) is also different - she is attracted to the fisherman because of his song, and brings him treasure, as well as in due course bearing him several sets of twins. No question of him keeping her with him by coercion. They are not separated by any twists in the story and are last seen in the story heading off into the sunset together.
My take on "Mary Chinidh" being connected to Mélusine? Bit of a stretch, linguistically. And the story is completely different. Mélusine marries a human who finds her (generally in the depths of a forest) and requires certain time to herself. One day the husband spies on her and discovers she's really a serpent/dragon/has a fish tail. She leaves him, often in a fairly dramatic way. There are other tales of a mermaid (French - sirène) who sings to attract a lover and then carries him off under the water. Generally a fatal result for the human. Neither of these story variants involve shape-shifting, seals or lost skins.
You can find literally dozens of selkie stories on line, and there are books on the Melusine tales. I'd also suggest, if you're interested, that you contact Mary (Dr Mary Mc Laughlin, who will certainly know more about this song than most people who contribute to Mudcat). (And I took several deep dives into the Melusine/sirène stories myself in the course of my own PhD research, as I had to deal with an underwater fairy).