Pretty Little Liars: how is "A" always 10 steps ahead?

Alrighty, since the first episode was very close to what went on in
the book, i'm going to take a wild guess and assume that the story
line stays relatively the same and that the only things that are
different from the show to the book series are things that don't
change the main story anyways..... so, my question is,

1. What mental illness did Courtney have that institutionalized her?

2. When Courtney's body was found, why was Ali (thought to be
Courtney...) not invited to her twin sister's funeral?

3. How was Ali released from the mental institution? Or did she
escape?
and while she was away how did she always knew what was going on when
it happened?

4. Above all, what happens to Ali (in the book series) after everyone
knows what happens? does she get arrested for killing her sister?
Return to her family? or do the cops never find out?

PLEASE PLEASE PLEASE!!!! i'm dying to know!