Evelyn still has the star stone, and you will eventually be able to acquire it.
Main plot quests were designed to be able to be completed even if you kill everyone (and with very few exceptions, you can kill everyone... though of course will miss a lot of side quests, and the solution to main plot quests may be more difficult).
Thank you both for the responses.
Just one more question. Is there a way to check something like:
"You have x of the 12 star stones". Something that would show me what I'm missing.
I'm at level 20 and struggling to figure out how to progress.
