With regards to the ridiculous rule about banning the ocean, if Ariel knew she was going to miss Triton so much, why didn't he just appoint Attina (his oldest daughter, for those of you who don't know) the new ruler of Atlantis, transform himself into a human and live with Ariel, Eric, and Melody?
Just because Triton will miss his daughter doesn't mean he's going to give up his responsibility as the king. He knows he has an important job. And he may not be sure of Attina's leading ability.
That idea is just too stupid. The best he could do to is to stay in contact with his daughter, son-in-law and granddaughter by using his magic to create a communication sound in a shell. That way, once they've captured/killed Morgana, they will be informed ASAP and Melody will be able to safely visit her grandfather whenever she wants. Unless it's possible for Ursula to return one of these days, but not as herself...
Why didn't Ariel and Eric just tell Melody why she couldn't go into the sea? Not only would Melody know why they have a giant wall, but then she'd know that there is a threat out there, and wouldn't be putting herself in danger everyday by going into the sea, and not knowing. If they kept the locket at the beginning (or told Melody when she found it), she'd believe them. Is it because they don't trust her with the truth? They seem to trust everyone else in the kingdom to keep a secret, so they should with their own daughter, especially since this whole thing concerns her.
And this is exactly why the film is viewed as such a prime example of Sequelitis.
When Morgana had the trident in her hands,why didn't she use its power to achieve One-Winged Angel and grow into a giant enemy monster version of herself like her sister did before???
Because she's insane and an idiot. Morgana's goal was to only show her mother that she can be just as good as Ursula, and mentioned that she was The Unfavorite out of the two.
Another one.Sure,Morgana's ultimate punishment was a Fate Worse Than Death combined with And I Must Scream,but,wouldn't it have been better to blow her up to tiny squid bits to get rid of her once for all?.What if some random idiot (or worse,another villain) finds her and releases her from her icy prison?
Similar to one of the above questions, how did Ariel conclude keeping Melody ignorant of her heritage was best? Not allowing her into the sea is one thing, but where was the harm in telling her about Ariel's origins?
She probably thought knowing mermaids existed would make her want to go in the ocean more, since it'd effectively be telling her there's an entire country full of sentient beings who love swimming as much as she does. Or she might be worried Melody will notice the Generation Xerox and think she's a hypocrite. There's also the possibility the knowledge there are other beings down there will make her view the ocean as safer than she should (e.g. "I'll be fine; the mermaids will protect me").