Well, it's actually Interaction which states that Jane Jensen took over quite a bit of the game, and wrote out 'writing, dialogue, and narration', when Roberta left.
It was Roberta that admitted that the style was more Jane Jensen, than her own, in that quote I gave above. That quote was from the The Official Book of King's Quest VI, as I recall.
Has anyone transcribed the interaction articles? I have many copies I'll have to go read through them again. Behind the Developers Shield and Inside the Chest programs has some interesting behind the scenes information as well.
These are quotes from one InterAction article, around the time of KQ6;
"I originally wanted to be the Creative Consultant on King's Quest VI...I wound up being much more deeply involved than I planned...As hard as it is for people to imagine a King's Quest game that wasn't designed by Roberta, it almost happened with King's Quest VI".