Just to give a (relatively) short summation:

It sounds to me that the stereo and mono mixes are the same person, just different takes. Rinzler's book says that sound mixer Derek Ball went to Fraser's house to do ADR, so I am convinced that both mixes are her.

Fraser has a British accent, so I'd guess that during shooting she either used her normal voice. Even if she did put on an accent, Lucas clearly wasn't satisfied, and dispatched Ball to loop Fraser's lines with the familiar American accent.

When the Dolby Stereo/70mm mixes were assembled, Lucas/Burtt/whoever selected one out of the multiple takes for each line. Later, when the mono was done, they changed their mind and picked different takes for every line. (I could go on and on about how Lucas and/or Burtt kept swapping out ADR over the years, but I won't.)

As to "tell Uncle"/"tell your uncle", the shooting script says "tell Owen", so perhaps that's what was said on set, and Lucas changed the line for the ADR script. "Tell Uncle" could have been Burtt trying to get the ADR to sync better with the footage.

Perhaps her lips could be more clearly read on a theater screen, but since it's nearly impossible to read them from that distance on a TV, maybe Matthew Wood thought "screw it" and went back to "Tell your uncle."