A simple question. At the end of Mr. Nobody Nemo decided to abandon both parents, how is it possible that he met Anna in the future? I can't figure out how he could've met Anna since Anna will be living with her father and new stepmother happily since her father wouldn't have to deal with Nemo's rebelious acts.
The thing is that we don't know what happened after he left both parents. Perhaps he reunited with his parents years later and met Anna during a visit. Perhaps he met her in a totally unrelated event, like he did in the spaceship. The details don't really matter, and the movie, wisely, doesn't dwell on them. The theme of the movie, choice and consequence, is intact either way. Nemo makes a choice and it ended with him reuniting with Anna.
All the possibilities were the attempts to find the correct decision to his impossible choice, following each choice to its conclusion. They were figments in the mind of the 9-year-old Nemo at the train station. Eventually he takes a third option, to make neither of two choices towards an unknown future.
Consequently, his life simplifies itself, and as result he is able to return to his childhood, watch his parents get back together and be with Anna. The 118-year-old Mr Nobody realizes that his younger self finally found his true love and life.
Source: Mr. Nobody - Epilogue at Wiki
Given his gift of knowing the outcome of his choices, I'm sure he finds his way to meet Anna in the future. The same way as he was able to get into close relation with each of three girls sitting on a bench, his possible future wives.
All of his lives were real. What started it all was that point at the train to decide to go with his mother or his father. It is unknown which timeline he chose to lead his 118 years old self. Two things were certain: he loved all his lives and he loved Anna the most.
When the Big Crunch occurs, he was laughing because he get to do it all over again. It was hinted that he did this before (assumingly to be infinite). Since his memories were not erased, he get another go at it. Which choice will he pick this time?
And his didn't reunite with Anna (though he can if he wanted to). It just showed two things: the time was repeating again and Anna was the girl he loved most.