We missed a few things

You must have seen the cut version...the version shown theatrically to U.S. audiences because apparently the U.S. was thought to have preferred a happier ending...which is just ridiculous on so many levels.

In the film's uncut and very much intended original ending, Sarah turns in the jeep to see Juno's bloody apparition sitting next to her. She freaks out and wakes up lying on the cave floor in exactly the same spot she had just landed after falling down (when the daylight seems to wash over her from out of nowhere). So she wakes up, curiously forming a kind of animalistic stance where she is down on all fours, looking around, hearing the sounds of crawlers and very much resembling one's pose. Then she hears her daughter whisper "mummy..." And the birthday cake appears to her, candles lit, daughter sitting in front of it smiling. Sarah looks from birthday cake to her daughter and also begins smiling. The camera then pans out and we see that the light we thought was coming from the birthday cake is actually the flame from her still burning torch and she is hallucinating. We are supposed to get from this that Sarah is no longer concerned with survival, has made her peace with death, and has finally crossed over into full blown delusion. The camera continues to pan out revealing Sarah is actually hopelessly far underground, the collective growl of crawlers increases and culminates in a full blown scream and the film ends. Cut to the picture of the girls and the credits.

So the escape was just a dream and while that might seem manipulative, or a cheap trick, I like to think the director intended for audiences to see how screwed up she would be for the rest of her life . Sarah's guilt manifested in the form of Juno's ghost and that was merely a foreshadowing of how haunted she would be for the rest of her days. So the seemingly more depressing ending of dying in the caves is paradoxically the happier resolution for Sarah.

As for the affair: it can be strongly sensed when viewing the opening 5 minutes a second time. There are all sorts of body language cues and weird looks going on. There are many other suggestions. Juno's pendant says Love Each Day which is something Sarqh's husband said to her frequently. Juno kisses this necklace for good luck prior to attempt to cross the chasm. Juno also leaves Sarah in the hospital after the accident rather than consoling her, probably because she couldn't deal with the conflicting emotions of being so distraught over Paul's death and feeling guilty about the affair.