Hi. Ccan you give us more explanations about... why you thought we need to clear this point? Because yeah, there is two pages: Evil Queen and Ursula; so I don't really understand why you think we need to discuss about it ^^
And no, the "onceverse" of the Miller's Daughter is Cora not Regina ^^
Ursula is the Sea Witch, I agree, but Regina still masqueraded as her, which is why it's noted on Regina's page that she played the key role of Sea Witch with Ariel within The Little Memaid fairytale. This is to show Regina, not Ursula, bargained with Ariel to keep her legs rather than the real Ursula doing so.
Agustinaldo wrote: "Ursula" IS the Sea Witch. The Sea Witch is a separate character from the Evil Queen in the Onceverse.
Ursula is a sea goddess, and a neutral character during the episode (similar to her original role in the original fairy tale). While Regina takes the place of the person who steals Ariel's voice in exchange for legs, essentially making her Ursula as well (with Regina's Ursula disguise being more closer to the iconic Ursula). The only thing messier is how Anna is based off of Gerda from the Snow Queen fairytale while her mother is as well on Once.
Regina IS based on the Sea Witch from the Little Mermaid fairytale in that she assumes the role the Sea Witch had in said fairytale: tricking Ariel into giving up her tail for legs, and then taking away her voice.
The fact that there is The Real Ursula out there, and that she is based on the Disney version, doesn't change the fact, because, as I always like to remind people, Disney movies ≠ source material fairytales.
Side note: the fact that two characters are based on a single one is not surprising: both Anna and Gerda are based on Gerda from the fairy tale, and both Elsa and Ingrid are based on the Snow Queen. But if we want to dig deeper, both Aurora's Mother and Aurora herself are based on the Sleeping Beauty, both Cora and Cinderella are based on the Miller's daughter (because Cinderella functionally got the Miller's Daughter's role in The Price Of Gold) and so on.