I've been eagerly awaiting the fic that will give a good explanation for
the way the Senshi eventually take over.  This wasn't it.  They just up 
and decide to take over by force for no reason at all?  There should at 
least have been some kind of major international crisis that sparked 
their play.  My suggestion?  She takes over after a limited nuclear
exchange or during a crisis that looks like it might turn into a full 
nuclear exchange.  

Once again, the practical issues of "purifying" absolutely everyone in 
the world leave me skeptical.  Zapping billions of people one by one is
going to keep Serenity pretty darn busy, busier than she'd like to be, 
particularly since she'd have to do the same thing to every child 
subsequently born if she wanted to make it stick.  
Zapping the entire planet en masse, apart from not allowing them the 
choice of exile or purification, seems a bit over the top.  The original
Serenity if she'd had that kind of power wouldn't have been forced to
destroy the life in the Solar System at large. 

Oh, and one more thing:

 My name is Kenji Shiromatsu, and I am a second-year graduate
student at the University of Kyoto... or, at least, I was, yesterday.
I am twenty-four years old, wear reading glasses, and am not of any
distinguishment physicality-wise

"distinguishment physicality-wise"?  Pretty ugly.