There is no way that the Mono Mix is just a different take. That’s a whole new woman. Considering the rest of the audio errors the mix corrects, I bet they decided the original voice was just a bit off-sounding for her and decided to redub her entirely.
That makes me wonder something else… Considering the sheer amount of new ADR in the Mono Mix, is it at all possible that there was another recording session that took place after release?
There are some pitch differences, but the voice is the same. Since you bought it up, I even double checked it with her real voice (She was a principle in A Family At War which is out on DVD and some of the episodes are currently up on YouTube) and it is all the same person. The unique tones of her voice match in all three. And it isn’t just my ear, but several reports on the ADR sessions conducted by Ben Burtt confirm that it was Shelagh Fraser doing an American accent. Her American vs. British accents are very similar to Hugh Laurie’s.