Had Melody not run away at that exact time, her relationship with her mother would have been ruined beyond repair.When she found out about her heritage, she was angry enought to give Morgana the trident, despite her mother's clear desperation and warnings. The only reason they were so easily reconciled was because they were facing mortal danger, and in the face of possibly losing your family to death, all previous squabbles seem trivial. However, had she not run away...
Only those with mermaid blood can understand sea animals.It would explain why the kingdom's children laugh at Melody when she talks to Sebastian.
Only men are allowed to rule in Prince Eric's kingdom.The other children openly laugh at a princess. At best, that's no way to court royal favor. At worst, they could find themselves in a very bad place after Melody becomes queen. Unless she can never become queen. Unless she must marry and then lose any influence she has to her husband...
The merboy and young man that Melody meets at her party are one and the same.Think about it, both have blond hair, are about the same age, and may have feelings for Melody. The merboy may have seen Melody out swimming but was unable to see her face, but when he saw she had feet, he knew she was a human and had to meet her, what better place to do it than at a ball? He made a deal with some sort of magic practitioner who turned him into a human (temporarily, provided he payed the magic person with his memory of meeting her, and judging by what happened I'm sure he was happy to lose those memories hence why he didn't remember seeing Melody.) "What about his British accent" you ask, he obviously faked it. How did he know so much about human stuff, like dancing? The same way Ariel knew about it. And I know, I know "But merboy has green eyes while dance partner has blue eyes" perhaps another part of the magic persons spell.
Eric is actually Prince Charming.This is why he's notably more passive in this movie. At the same time, Eric cut his hair and went over to Cinderella's castle just in time for her story to get a bit more action in it.