I'm not sure how to send a PM either! Hmmm... Well I can say this:
There's two different endings for each choice on the major last choice, one for low affection and one for high. There doesn't seem to be a way to do spoiler boxes in the comments either! What a pain... Feel free to email me if you'd like: Shuta.Medu@gmail.com or if you have twitter you can contact me there ( https://twitter.com/MakioKuta ) and I'll gladly help you out in greater detail.
Vaguely speaking though:
Free - low aff = Beach Ending
Free - high aff = Office Ending
Lab - low aff = Free Ending
Lab - high aff = Lab Ending
Affection refers to the mermaid's affection for the researcher. Again, I can go into more detail if you want over email or twitter DM!
Thanks for playing the game; I'm sorry that one ending has you stuck this long.