Compare and contrast
I would teach social skills to both "groups" -Nancy, and Cathy and Carl- and they seem to have no clue about what others may feel and what their behavior shows about their emotions (frustration, anger maybe...); or, maybe they are aware of what is going on but they deliberately choose to ignore it. I think that would be good to have a car picture of what is at play there since the disruptions seems to be continuing. In both situations, I would review the classroom management rules, consequences, procedures etc. The reasons are dual: first, to remind the "offenders" that they have to respect the rules like everybody, and second, to show the rest of the class that the offenders will not go free to act as they want. The danger is that, if the class sees that their misbehaviors are tolerated, then they would also try and play with the rules when they fell like it. I would not want to lose control over more students.
Finally, I would try a different approach: I would try to establish a visual cue with Nancy to remind her that she is monopolizing the discussion, to see if that would bring more awareness than just a reminder - seeing on the board or on a paper how many times she repeats the same misbehavior might make it more real and 'shocking', even more so because she seems to care about her studies and be willing to be a good student (she studies hard).
As far as Cathy and Carl are concerned, I would simply separate them to see what happens when they are not together. Do they talk with other students? Do they create others disruptions? Do they finally listen (hmmm, one can dream???).
For bothe groups, I would use a light tracking system such as self-monitoring irst, as then a Daily behavior form requesting the signature of the parents.
Growth mindset
Teaching social skills, individually or with the class through games and talks for instance, would allow to bring some more growth mindset in these cases, by learning and practicing the positive behavior instead of punishing constantly the bad ones.
Going through the classroom managament with the students could help update them if need be, and allow to start on a better ground with tools that match the class' needs.
