Translation:She's not there either.
I'm just a beginner, but I think 那儿 is Beijing dialect. In other parts of China, they say 那里. As far as I'm aware, they're interchangeable.
They are both used throughout China. 哪儿 is more common overall, especially in the northern parts.
There's no difference, except 「哪里哪里」 can be used as a polite response to a compliment.
Does anyone else hear a "pa" sound instead of "ta" when then sentence is fully read and hits 她?
Well, she's also not there should be correct. 在 doesn't always mean inside
Both "She isn't here either" and "Neither is she here" should be acceptable, but "She isn't here neither" is definitely non-standard.
Personally, I would see that double negative as additional emphasis that she isn't in a particular location, like "You say that she's here, but I'm telling you she isn't!"