I didn't get it from reddit, I got it from ONTD
Ariel and Eric's casting is
here
(scroll down to comments)
And Triton's is
here
(scroll down to comments)
This person has a friend who's dad is a producer on the movie. In June when Zendaya was shown with red hair in London they had stated she was talks but wasn't anymore. They also stated that Disney did consider Gaga for the role of Ursula as well as actual drag queens.
I think Eric was suppose to be the thing that "broke the camel's back" when it came to Ariel's fixation with humanity, like he became the physical embodiment of that desire to be human but it didn't come across that way b/c these animated movies lacked a certain complexity to them.
And I agree with your Eric casting comment in regards to Harry Styles.