All I remember is the entire audience laughed at what was a very meaning line
It's a weird line without the context, like with a big chunk of Ikuhara's work. Although it's nice to see something basically say that this movie really does serve as a giant showcase as to why Usagi is the most qualified to be the leader, and more than just an incompetent crybaby like most of her detractors accuse her of being.
Usagi is definitely one of those characters who, I think, suffers from "slow growth." She definitely does grow as a character throughout the entire series, but if you're only watching it episode by episode, it's easy to lose sight of her how she's changed and to think she's just a goody-two-shoes crybaby who suffers from a bad case of "main character syndrome" (i.e., not having anything really particularly interesting, but always being better than everyone else... just because).
All that negativity out of the way, though... I don't think that's how she is at all. Rather, I think that she really DOES grow a lot throughout the series... it's just really slow. Kind of like how you never notice your own kid getting taller, but if you look back at a year ago, or even six months ago, they're totally different. Since Usagi is the focus of the series, they can't really have too many episodes where she changes dramatically, but if you look at the character you first see in episode one, struggling to try to save her friend, and the Sailor Soldier who stands up against Wiseman two seasons later, it's pretty obvious she's grown into her role.
Clumsy though she may be, I think she grew out of "incompetent" pretty early in the series, no matter what people may say. =p
As for Xenian?
From reading Ikuhara's quotes, I get the impression that there is one Xenian (i.e., not a species), and that she is trying to spread herself throughout the galaxy.