I thought the prevailing theory was that while Beru’s voice was dubbed, it was still Shelagh Fraser, just doing an American accent, and with the mono mix being a different take from the same dubbing session. Doesn’t J.W. Rinzler’s book, or somewhere else, mention an ADR session with Fraser?
The stereo and mono mixes do not sound like different people to me, just different takes of the same person (whoever it is).