It's been known that sometimes a person will fall for someone similar to their parent. Notice that Alex, the blonde merboy Melody befriends, has a Nice Guy attitude like her father and also sports a green tailfin like her mother (when she's a mermaid).
Notice Melody's reaction when the merteens invited her to hang out with them: She's speechless. Then, remember her (initial) with the human teens. This is perhaps the first time that Melody has felt a part of something and could be with people like her.
Why was Morgana so good at manipulating Melody? Aside from Morgana being an expert at manipulating and Melody being naïve, there's also the one thing they (initially) have in common: Mommy Issues.
Notice that Melody has accidentally bumped into blondes of teens from both of her heritage: Human and merperson. When she bumped into the snooty, blonde girl you can see that they have met before. When Melody bumped into blonde merboy, he didn't know her. In a way, both of those times symbolize something:
The meeting with the snooty, blonde girl represents how much Melody is depressed about her life as a human teen, since she's so different.
The meeting with the cute, blonde merboy represents that Melody can live a new life and be happy and accepted.
The wall. It was built to make sure Melody never ventured out into the sea. Ariel had a valid reason for doing so, but Melody, obviously, didn't know. In a way, the wall also represents (to Melody) that Ariel is keeping her daughter from being herself.
Melody is a mer-human (half human-half mermaid). She is the physical representation the union between two different species. And at the end, Melody is the one to destroy the wall- the wall that separated these two groups for 12 years. The first mer-human reunited the union between two groups.
Yellow is the color symbol of happiness and joy. Notice there are three yellow things that Melody is connected to in the end of the sequel:
The yellow dress- The happiness she finally have at being herself and not having to feel "weird".
The yellow locket- The locket she received from her grandfather represents the happiness that she and her grandfather can have together as a family.
The blonde merboy- The (potential) romantic, happiness Melody can have with Alex, the cute blonde merboy. His face just confirms that he likes her too!
Why did Melody faint after being caught by Dash, even though a second earlier, she was wide awake? Let's look at the list of what a 12-year-old girl, with a slim build has been through: Nearly died drowning, went running and climbing a castle by an evil sea witch, caught and desperately held onto a very heavy weapon, and again she faced a near death by being pushed by the same evil sea witch. It's been known that strenuous activity can cause a person to literally fall asleep or faint when they land on something soft.
How was Melody able to climb Morgana's tower so easily? Well, for about most of her life, she's been swimming in the sea and afterwards snuck back into her the castle. By climbing a window. Possibly everyday. There's also the fact that her mermaid genes give her some impressive physical qualities.
There's also the fact she inherited Super Strength from her mother's mermaid genes.
A bit of Fridge Brilliance regarding Morgana's status as a Suspiciously Similar Substitute to her sister...it may well be intentional on her part. She lived most of her life living in her sister's shadow, so it's very likely that she started emulating her, either actively or subconsciously in a desperate attempt to gain her mother's favor.
Mermaids are fish from the waist down, which implies they fertilize externally. For all of Ariel's life, she probably understood having children as a matter of squirting into a hole. Now imagine her reaction to going through humans' messy and painful way of doing it.
Also, consider from Ariel's POV, the only difference between the bodies of humans and merfolk were the former had legs and the latter had tails. She probably had no idea humans also have genitals. It's not a stretch to imagine for a while, she was fascinated with the organs between hers and Eric's legs. And then, Eric tries to show her just what those organs are for...
While they are fish from the waist down, they are human from the waist up and clearly have hair, belly buttons, breasts, and nipples. So they are very likely mammals, despite having a tail. It could be possible that, like dolphins, they have their genitals hidden inside of their bodies and essentially the mating process is the same.
There's also a scene in one episode of the TV prequel series ("Against the Tide") where an aquatic mammal gives birth right in front of Ariel, and her reaction amounts to, "Yay, babies!" So she's definitely familiar with mammalian reproduction. And given how she casually walked around bottomless in the first film until Scuttle told her she should wear clothes like a human, she probably views genitals with less interest than the humans do. The real danger is that she'd be in public when she suddenly discovers what else you can do with those parts.
Morgana, at times, appears to be even more deranged and depraved than her sister. Now think back to the way Sebastian described her—as "Ursula's crazy sister", plus Triton flat out calling her a "madwoman". That's right, next to the slimy con-artist who turned countless merfolk into helpless polyps when they failed to honor her fraudulent deals and tried to overthrow King Triton twice (once using his own daughter as a tool), Morgana is seen as the crazy one. And given some of the things she does (like gleefully try to feed an infant to a shark), it's not hard to see why...