This is a good topic for a thread.
My opinion is that both are dubs. My initial instinct is that neither is her real voice, but maybe it is.
My reason is this: she is the only actress dubbed in her scenes. Contrary to what you wrote, production audio is not 100% useless. Back in the 70s there was very little dubbing, and you used as much on-set as you could. This is why older films sound "rougher" than today, because today much of the dialogue--whether in studio or on location--is dubbed.
Very little of the Tunisian dialogue is ADR'd from what I can tell. However, a lot of the studio dialogue is. The entire control room conversation between Han and Luke ("she's rich...") is almost 100% dubbed, probably because the stormtrooper costumes clattered so much.
In the case of Beru, all the scenes she is in have production audio for at least most of the scenes. Owen is on-set recordings. Luke is on-set recordings. But Beru is dubbed. Always. There is absolutely no reason why her closeups at the dinner conversation should be dubbed. So that tells me that the only logical answer was to give her a different voice. Probably because she had an accent and it seemed out of place since the rest of the family is American sounding.
It's interesting that the book says she recorded ADR though. Maybe the voice(s) is actually two ADR recordings of her putting on an American accent over her original recording. I agree that the mono is the same actress as the stereo--maybe she was re-recorded in mono because the original stereo mix was so obviously dubbed (although you can still tell in the mono). Or, maybe they initially tried to have her dub herself for the same reason, but realising it didn't work or sound right got a different actress for the final mix.