I agree with you on Shasta.
The narration also noted that Rita continued to be able to see the "alpha mimics" even after dropping out of her loop, if I remember correctly. I guess whatever happened that linked them to the alien's tachyon communications changed them in a permanent way.
One thing that threw me about the ending was that he doesn't seem to consider the possibility of facing the same choice as Rita.
I recently read the manga version of this book at the local library. I'm not sure I really understood the ending, or why Rita actually tried to kill Keiji. It was odd. I usually like odd, hell, I live there, but that was too odd for me.
Based on the book, only one of them could survive the battle. As long as both of them were alive, the one who didn't score the final kill would act as an antenna and pass back the information that the Mimics needed to survive the fight. She attacked him because they were both consummate warriors who knew nothing but battle by virtue of the repeated cycles. Thus, the only way to determine who would survive was to find out who was the better warrior for humanity by means of mortal combat before striking the final blow against the Alpha.
Currently reading the original LN that was most likely fan-translated based on where I'm reading it, and found this passage:
In the middle of the testosterone display the lone woman stood out like a sore pinky. She was a tiny little thing standing off by herself a short distance from the rest of the squad, something seemed out of whack.
[...]
Suddenly, it hit me. That's Rita!
[...]
Rita Vrataski was the most famous soldier in the world. Back when I signed up for the UDF, you couldn't go a day without seeing the news feeds sing her praises. Stories titled "A Legendary Commando," "Valkyrie Incarnate," that sort of thing. I'd even heard that Hollywood was gonna make a movie about her, but I was already in the UDF by the time it came out, so I never saw it.
Hilarious in Hindsight considering the movie.
I recently listened to this one on audiobook. Based on my reading through the description of the original light novel, it sounds like it's an accurate adaptation rather than the changed plot for the Tom Cruise movie. I enjoyed it. It took me a while to figure out the time loop setup, and I'm still a little hazy on how exactly the whole "antenna" thing worked out (It makes sense that only one person could be looping since the loops are essentially precognitive dreams. But once you drop out, why would you be special in terms of sending the signal back?), but it was an entertaining premise, and created a memorable dilemma for the characters involved.
And is it just me, or was Shasta a very animesque character? From the exaggerated physical proportions to bonking her head on the door repeatedly and each time dropping to a crouch to rub her head, I half expected the text to mention a huge sweatdrop by her head when she was nervous.