what's in the end?

In my opinion the "person" who comes back from the shimmer isn't lena. an argument could be made that Lena's replicant as the only person who can give account of the events that took place on the shimmer is an unreliable narrator. The Lena's replica can manipulate the story to her desire. The events she recalls could be remnants of Lena's memory that the replica has assimilated. when her husband came back from the shimmer he seemed not to remember what had happened, but how did he find Lena's home and how did he recognize her?
my guess is that this replica had assimilated some of his memories not all buy some. therefore it would not be impossible to assume that the same had happened with Lena's replica. The ending of the movie also seems to point this out. We see that "Lena" has a snake tattoo, this could mean that the replica has captured more than Lena's DNA but has integrated some of the DNA of the other woman who were in the mission.