My Mother's Memoirs - annotation, consolidation of diaries

My mother wrote constantly.  When she wasn't writing letters she  wrote diaries, and when she wasn't writing diaries, she wrote memoirs.  In fact, I have come across 4 different memoirs, from 4 to 30 pages long, with much overlap.  The fascinating thing is that each time she told the same story, she told it from another point of view, with a different emphasis, thus adding more and more detail every time.

The question is, how to write this up.  I have already transcribed her writings verbatim (from handwriting to computer documents), for surely they must be preserved just as she wrote them as source material. 

But there are factual errors, wrong sequences, wrong dates -- verifiable -- that I would like to correct.  There is too-chatty style that I would like to change.  And there is historical context that I would like to add.

In short, I need some "rules" to follow in telling my mother's story.  Does anyone have any suggestions?