In the cast Sonoya Mizuno is listed as Humanoid & Katie (Med Student), why is she listed as this?
Did the whole 'shimmer world' destroy itself after Lena killed 'herself/clone'? If so why didn't it happen after any of the others die.
Presumably the actor appeared as herself in Lena's lecture, and did motion capture for the alien clone.
When the others died they had not been cloned, their bodies were being changed by the shimmer. Lena's clone was part of the alien being, presumably it was destroyed by the grenade.
All of the mutations/changes inside the Shimmer are controlled by the alien thing in the lighthouse. And the alien always directed things to reflect/refract upon one another. So when the alien made itself into a clone of Lena, it was unwittingly giving Lena the power to control everything inside the Shimmer.
All Lena had to do was transfer her intentions/thoughts to the alien/clone and the alien/clone would mimic them. When Lena attacked the clone/alien, the clone/alien attacked back. When Lena fell to the ground dazed, the clone/alien fell to the ground dazed.
What I think happened was that Lena finally realized this. So when she gave the clone the grenade, she intentionally had suicidal thoughts, hoping the alien/clone would faithfully copy those thoughts. And it did.
And since the alien/clone was also in control of the Shimmer at the time, it not only self-destructed itself, but it also passed on those self-destructive thoughts to every mutant thing in the Shimmer, leading to the entire Shimmer's destruction.