I can tell you for sure that there is no extra dialogue in any of the code that implement the Scenario feature, in fact the opposite is true: There is some skipping of dialogue for better "continuity" between the scenes in the scenario playthrough.
The "final 10%" barrier will be hard to complete. This is due to many subtle dialogue variation branches, often based on choices made 2 or three chapters previously. The dialogue in these variations is often quite similar and have many lines that are identical between the branches, so there's no easy way to see which branch you are seeing from the player perspective until you hit the difference.
More than any other game I've ever seen, NIF is implemented so that previous choices do have effects all the way down the line, at the dialogue level if not at the route level. It's an accomplishment that I wish more players would appreciate rather than being grumpy that their specific kink is not catered for in more detail. MrSL is but one man, battling the many headed hydra that is the combinatorial explosion a truly choice-responsive game represents.