I think you make a good point about Carla giving up her life on land. I’d say the same thing for anybody who ships Marzel with Elena or Naomi. They also have hobbies, friends and family that they would miss. Unlike Ariel, the girls never show dissatisfaction with their lives on land. Ariel can be transformed into a mermaid to visit her friends and family under the sea whenever. If one of the girls chose to give up their legs, they would become a Sirena permanently.
Exactly. Plus, they have Master Royal Wizard Mateo, who I believe would 1000% be able to create a shapeshifting spell/object that was only temporary if it were so desired.
It's interesting you bring up “The Little Mermaid,” I believe Ariel would have chosen to live a life on land whether she had a crush on Eric or not. It's just that everyone both in and outside of the movie assumes that she ONLY did it for the romance, which just isn't true.