I think you need to go ask yourself and your characters all of these questions instead of asking us for every last detail. Part of being an author is creating your own world, and you will never learn those skills if you have other people do all the work for you. A lot of these questions are so trivial that you and your characters should easily be able to figure out the answers. To me, it sounds like you don't actually have a world in your head, you just have this vague idea of one and are trying to approach it too logically, too formulaically. You need to get a good idea of what this world is going to look like for yourself. That said, have you thought about the... erm... mechanism for reproduction as it pertains to having a mermaid mother and a human father? As far as I know, mermaids don't have... lady parts, and since they are fish on the bottom, they'd likely reproduce like fish do -- the females lay eggs, then the males come and fertilize them after the fact. Is this how the human males would do it too? It seems quite vulgar, and not nearly as romantic as you probably imagined it. If that's how you want it to happen, that's fine, but just keep in mind that my initial thought about them having human fathers was that the backstory was creepy, not romantic.
I vaugely imagined mermaids reproducing like mammals. And them having human fathers wasn't really supposed to be romantic, more of a one night stand on a beach thing.
