Wow... seriously... I know this is a topic about theories...but jeez...
...the writer's are tricky but there is no way that they can be true to the story and the characters if he has already met your mother.
I think the key to the storyline was in the New Years Eve episode where he says after he steps out of the limo to not feel sorry for your father... and that this is all part of the story. Robin/Victoria/Barney/Marshall/Lilly/"Wendy"/and whoever else is in the story make up his life.
He is recounting how the actions you take, the friends you make, the experiences you undergo make up a lifetime. And while he is concentrating around that central theme of meeting "the one" that is not the only point.
And please people... stop getting caught up in the kids and the future setting... There may be a few clues there but think of how little the kids actually say. And stop trying to read into it. Why does their mother really have to be dead for him to be telling this story. He is in his 50s right, perhaps he is finally sitting down and saying this is the way it happened... and you can bet that he isn't telling the story word for word as the show happens. Just let it happen.
He will find the mother eventually, but we have to watch how he becomes the person he is as he follows his path.