Can someone explain the ending to me?

I thought this movie was awesome up until the very end, where everything seemed to suddenly go off the rails. Specifically...

1) What caused all the old lovers to rise up against Miriam and how were they even able to?

2) What killed Miriam? The fall? Is she just as susceptible to dying from an injury like that as a mere mortal would be? If so...

3) At the end we see Sarah in the future with a young woman and, presumably, a coffin of an ex-lover of her own. How did she survive her own suicide if the answer to #2 is "yes"?

Also, why did John seem to think he was going to live forever young, as repeatedly indicated by the dialogue throughout the film, but then indicate that he knew about the rapid onset of old age by asking how long it took for the others once his time came?