Wasn't the movie about luring Ariel though? I thought they wanted to kill her or something.
TLM 2's plot is that Ariel lives on land with her family, and her daughter feels out of place or something, and she turns into a mermaid (Ursula I think....) and she can't really change back. I don't remember the entire plot for the second one....
Morgana was trying to get revenge for her sister, Ursala, and she wanted to rule over all the seas as Ursala did, but she could only do so with the trident. Morgana doesn't use anyone as a bargaining chip but rather tricks Melody (Ariel's daughter) into giving her the trident in exchange for her true identity (becoming a mermaid, essentially the opposite of Ariel in KH1). She captures Ariel after she gets the trident only because Ariel overhears her plan along with Melody (Morgana tries to drown Melody as well) so that nobody can prevent her from taking over the ocean, although obviously in the end she fails.