I couldn't believe Nicole Kidman would appear in something so contrived, so I reconsidered the role of the coincidences. If I've stumbled on to what was intended, it's worthy of Charlie Kaufman or Philip K. Dick (is that good or bad?).
In religions, reincarnation is a punishment; another chance to atone and reach "nirvana." There is suffering for prior sins, but no awareness of previous "lives." "Birth," however, imagines that the coincidences (maybe written as "divine" or "psychic" interventions) enable the boy to slowly become aware of his previous incarnation, and then its sins. This time, his actions and sacrifice ultimately protect his ex-wife. In the end, she has forgiven the boy, although for a "trick" he didn't play. To a boy, the act of forgiveness perhaps would be enough, especially with the assistance of his psychiatrist, for him to resume a "normal" life. Hopefully any additional suffering caused his unwitting ex-wife wouldn't warrant "Rebirth: Sean 3."
Should movies be so confusing and complicated? Even Ebert couldn't come up with a single interpretation supported by all the "facts" presented. Is understanding a movie, like revenge, a dish best served cold?