Good insight about the use of flashbacks, RouxB!
I can't speak for Sarah, but, yes, I think you could say the whole story is kind of an Ennis flashback, in its "revised" version with the italicized prologue. It starts with Ennis "suffused with happiness" (what a wonderful phrase!) because he has been dreaming about Jack. Then it goes into their story, and at the end it comes back to Ennis dreaming of Jack.