@Klauthzer0
The way I understand it:
Shinobu convinced Shouko that she has the power to end this curse, which she did. So basically, Shouko has moved on. However, we don't know what happened to the other mermaids. Shinobu still has to bring back Shutarou and Yamaguchi.
What baffles me is why Shouko agreed to undo the curse on Shinobu's friends. Her chance at freedom is at her hand, but I guess it's because she didn't really start the curse on purpose and it has something to do with her family's lineage. That's why Shinobu was going to Shouko's hometown at the end of the manga. Shouko was a descendant of a mermaid who became a human.
I agree with you, lots of things are missing and there are lots of questions brought on the table on top of the old ones.