Allen and Madison are eventually caught while out in the ocean.
Of course the public was searching for them after they escaped. This disregarding the sequel, which is pretty much not canon. Madison came on land to kidnap Allen regardless of his feelings for her.
Madison's plan was simple. She was going to stay on land for a few days, seduce the guy, and then bring him back to her world where he could never return again. A few details:1. This is in line with how mermaids are traditionally portrayed.2. The original script is clear that in Madison's mermaid culture, she is told explicitly by the mer-elders that either she stays on land forever or the human has to come down there with them forever. The ending is more explicit that Allan becomes a merman when they swim off and enter the kingdom.3. If Madison stayed for more than six days, she would be outcasted by her society when she returned and forced to drift as a wanderer forever. This is what happened with the Sea Hag character (she appears in a deleted scene).Madison wasn't willing to gamble her life away for him. She also wasn't going to give him the chance to refuse her.She came to kidnap him and turn him into one of her species.
Allen and Madison's feelings for each other was just lust and not real love.
Pretty self-explanatory. Kornbluth had a crush on Madison.
Also pretty self-explanatory.