Question: Has the mother been cast yet? Also, does the film crew know who the mother is already?
Question: Out of Ted's kids in 2030, who is meant to be the older?
Chosen answer: In a flash forward scene in Season 8, it shows him with his first born: a daughter.
Answer: Ted's daughter is older as revealed in a flash forward in 'Trilogy Time', when Ted is with his then-only child: a daughter.
Lucky Penny - S2-E15
Question: I'm confused, what exactly was wrong with Barney's legs in this episode? Why couldn't he move them?
Answer: After running the marathon for Marshall with no training, he's allowed to ride the subway for free. While sat on the train, the muscles in his legs start to seize up after the marathon to the point it is painful to move his legs. This is why he can't move them. It's the same reason your body aches after a good session at the gym the day before.
Answer: Barney ran a marathon without any training and over exerted his muscles. While he was able to walk after, later his leg muscles gave out and he couldn't move (or more likely stand). It is not uncommon when one works out too much to be sore the next day, even if they feel okay afterwards. Thus the saying "I'll feel that in the morning."
Chosen answer: She was finally shown at the very end of the last episode of season 8, continuing into season 9. The actor's name is Cristin Milioti: http://www.imdb.com/name/nm2129662/.
Knever