I rambled too much. My main issue was that I deleted the original NPC mermaids and was worried I wouldn't be able to get to the hidden mermaid island. Turns out, all that worry was for naught. It did sort of work in that the new NPC mermaids that appeared (former townies, I'm guessing?) became mermaids and their tails worked/appeared. I befriended one of these new mermaids and after several hours scuba diving with her and doing almost nothing, she finally led me to the mermaid island. Yay! Also, while I was waiting for the mermaid to talk to me about the island, I spotted a bottle on the sea floor which instantly unlocked Plumbob Island. I guess I didn't mess up my game after all! I just will never get to know those original merfolk.
Thanks for the reply.
If anyone else wonders what I did, I used Master Controller on the City Hall then Total Annihilation then Mermaids. That deleted the originals without tails. After about a week, new mermaids took their places and these mermaids had mermaid tails in all different colors.
