The Narrator is the Third Person Perspective:
I would simply write these parts in third person from the perspective of the narrator as the third person. So the limited omniscience is limited by what the narrator knows.
Since C and D conversations don't actually include A and B, A and B don't contaminate the story. You don't discuss A and B in the narration.
Occasionally the listener might insert themselves and break proper third person perspective, but you can otherwise mostly write it like the narrator is the writer. I would use a line or scene break every time this happened to make a clear separation between the narration and the discussion between A and B. Anything they need to discuss about the story takes place here, like out-of-place questions or fully omniscient comments.
* * *
A paused. "Little did they know what was going to face them as they went down that road"
* * *
Then use another to cut back to the narration.
This functionally would look like the classic "cut scene" where someone launches into a story in a TV show and the main scene fades out to go to the story. The best popular example of this would be "The Princess Bride" where A is narrating a story to B about C and D, but B occasionally inserts into the conversation exclamations of disbelief, excitement, etc. Otherwise, it's like you are watching the scene between C and D as narrated by A.