How might the reader's impression of Aunt Alexandra be different if Lee had written the story in 3rd person?

In 3rd person omniscient we would have been able to understand Aunt Alexandra better from the beginning. The author could have shifted to her point of view. We would, however, loose the development of Aunt Alexandra through Scout's eyes.Although Aunt Alexandra has her faults there is still much about her that reflects her brother. This isn't fleshed out until much later in the book. This is made all the more poignant because it is noticed by Scout.

I don't know what you mean?