I conducted a way for Carol to create the Dark Fountain in Chapter 4.
Instead of creating it in the slim window between when Kris and Susie leave the mansion and they enter the church
>Kris and Susie leave the Church for the Holiday Mansion
>Carol, leaves Town Hall and goes to Church
>Either via being able to see oncoming clouds in the sky, or just the weather channel, she is able to convince everyone that they should cancel Choir Practice
>Toriel and everyone else leaves
>She can now make the fountain unnoticed.
>Leaves in time to call Kris in the light world.
>Then, once Kris and Susie leave the Holiday Mansion, she can either slip into the Dark World shortly after, or before they enter depending on how long the player explores
>Toriel does not bring up Carol because she refuses to even use her name.
Plus pic related.