That doesn't matter as long as she gets general idea. From what I read only some of the flash backs are hard to comprehend and the rest of the story is pretty easy to comptrehend.This could serve as a tool to help her level up on her comprehension skills since she enjoys it so much.
I think that she should be allowed to read it when she turns ten in half a year. By then, I'm hoping that she'll have finished the Hobbit and Harry Potter and the Deathly Hallows, and maybe read a little bit more manga so she can get used to the "extremes". I can probably hold off on her reading some parts that I would have had trouble with and she should be fine. I'm re-rereading the series right now, just to check, and when she reads it I think I'll read it with her in order to clarify some things.
It's just my opinion but I think you are worrying to much. If she is really into the manga then let her read it since there is nothing in the manga that is a bad influence. There is not much difference 9 and 10 1/2 and if she is already watching scary movies then you have nothing to fear as she is already use to seeing more extremes then what is in the manga.