Quick first question. If the outer senshi never talked to each other, how
did Haruka and Michiru get to be lovers? (I'm assuming that's a
relationship that predates the current timeline.)

It doesn't predate the current timeline. The fell in love once they were 
reincarnated on Earth because they were entirely separated prior to that.
Also, it is specifically defined in the manga that Haruka can chose to
be EITHER male or female.

Actually, hom much of the first moon kingdom was ever shown in the manga or
the anime? Was anything ever really defined, or was it left kind of
nebulous? And what differences are there between the manga and the anime in
this respect?

It was defined more in the manga than the anime. If you want copies of the 
translations to almost all of the manga, let me know.

I've got an idea for a 'fic, but I'd really like to know how much was
defined and how much was left to the fan to fill in in terms of the general
environment of the first moon kingdom, like how the senshi interacted with
each other and the general culture of the time.

The senshi are all older than Princess Selenity in the past timeframe. Selenity
was born when the senshi looked to be around ~5-8 Earth Years.

read the manga ^^;

misa :)