@Reb024
"But of course that doesn't bring back all the other poor girls. What I don't understand is what made Shouko change her mind about Shinobu. She could have waited for morning and eaten her right then and there."
Imagine being an immortal mermaid living on the same unaging body, without any interactions with the human world, having to watch the curse you created yourself, involuntarily, mind you, turning more and more innocent girls into mindless monsters for dozens of years. She wanted to go back to being human to end it all and meet the one she loved, but there was another way of doing so, and that was accepting death. Nanae would have taken longer to die, sure, but without mermaid flesh, she would indeed die eventually, and they could meet in the afterlife just the same.
Instead of risking Shinobu running away, somehow, and losing her chance at ending everything before she turned into the R mermaid, it was easier to grasp what she could then and there.