How does Scott Fitzgerald create a reliable narrator in the opening of the novel? How is Nick Carraway described as a reliable narrator?Okay.. well...

I think you've started with some great ideas in your first
point.  I'd go with what you have there, but I'd change the final point you make about
"not giving first impressions."  Everyone gives a first impression - and even Nick
Carraway makes initial judgments on the people he meets.  I think instead, what you want
to say here is that he never relies on his first impressions as completely accurate, or,
he always gives people the benefit of the doubt, or, he is able to basically see people
for who they are and still like them.

Think about the
dinner party where he reunites with Daisy.  He's introduced to Jordan Baker - and while
he finds her attractive, in the same thought, he knows she will never be someone with
whom he could maintain a satisfying long term relationship.  In short, she's shallow. 
Keep in mind that based on the advice his father gave him as a kid, this characteristic
is completely conscious.  As a narrator - he tells us exactly who these people are
(immorality and ignorance included), but because he's so likeable and nonjudgmental of
everything, they end up trusting him.

Instead of using the
2nd idea about being wrongly thought of as a politician - I'd go to the very next
sentence: "Most of the confidences were unsought."  The politician line is in reference
to the fact that he seems to be the holder of everyone's secrets - not necessarily that
he was "respected" as a politician (and note, political positions
have always been respected, but even in the 20s there was corruption in politics).  What
Nick is trying to say is that, for reasons he cannot understand, people feel comfortable
telling him their innermost secrets.  People are always opening up to him.  He doesn't
ask for it.  He certainly doesn't put himself in situations to receive it - but it just
keeps happening.

Nick is one of those all around good
natured, but somewhat quiet individuals.  He doesn't do a lot of complaining.  He
probably enjoys having a good time but is never the life of the party.  In the world of
materialism and richness that he is part of he is not very materialistic.  He is not an
attention seeker in any way.  These are the small but important details that make people
trust and open up to him - and make him a reliable narrator.  He calls things like he
sees them, he is smart and experienced, but deep down, nothing really bothers
him.