Figured out what went wrong with the Mermaid girl, I had to click on the notes before touching the pumps and then do the pumps in alphabetical order, while ignoring D and going straight down to the body to be able to read the final note which will trigger the event where you holster your weapon and talk to her. At least, that's what I did to get her to show up anyway in chapter re-select. I still don't have an option to go back in time or whatever for the snake girl, not sure if the option even exists for me anymore. Maybe it's because of the choices you make with the guy in the cocoon, I choose the second option. Can't go back and pick a new choice now.
By the way, you can craft a ring now to give to your waifu of choice if you have their affection maxed out at 100. Best save beforehand to see all the different waifu dialogue, no scenes though for now.