Easy Answer to Ending
First the basics:
The doubles (E2 and S2) are the idealized versions of E1 and S1.
E2 is outgoing, assertive and apologetic. Everything S1 wants to hear.
S2 will be described below.
S2, when we first meet her crawls over and rests with E1. Her first words are "I love you". This shows that she is loving even in a non-sexual way (S1 sleeps with E2), while S1 is cold towards E1. S1 is always angry and frowning.
S2 in the morning is warm to E1, apologetic, accepting of blame and can admit wrong-doing. All things S1 would not do. The big difference is that S2 smiles...A LOT. Even when upset. To the point of giving a little head nod and smile. That is her "tell" (each person has a "tell" that gives them away). The red herring is the hair clip -that's not the tell, it's the head-nod and smile. Again, S1 is always upset and frowning.
There is a role reversal that happens at the end.
By the end of the movie, E2 has become all of the bad things that E1 was (rude, selfish, combative). He tell everyone to *beep*-off and takes off on his own. He has had his reversal.
S2 has confronted E2 about his infidelity (he wants to leave with S1) and you'll notice her smile is gone, especially after the kiss with E2.
E1 takes S1 into the bathroom and gives his speech. he has become the person that S1 wanted him to be. That is his role reversal.
In the same scene S1, upon hearing the change in E1, loses the frown and gains a smile. A smile and even a wink/head-nod to E1 (there is a very distinct head-nod she gives to E1 - THIS IS VERY IMPORTANT). That is the "tell." You can now tell who S1 is by the smile/nod. You can identify S2 by the angry frown. And why not...her boyfriend (E2) cheated.
When it's time to pick between S1 and S2, S1 gives the same smile and nod to E1. S2 is dejected at the actions of E2 and knows she must stay.
Both E1 and S1 have grown as people, which is the whole purpose of the retreat. S1 even tolerates bacon for breakfast. She even says something to the effect "I think we'll have bacon" instead of "how about bacon again."
It's the song at the end, not the bacon (just like the smile being the "tell" not the hair clip), that is the red herring.
So in effect, S1 and E1 became the better versions of themselves (S2 and E2). Thoughts?