Based on the info we have, it seems like the first half of the leak was correct but based on questionably up to date information...and the second half--the part with Chrom, Shulk, and the Chorus Kids--was a guess. Sakurai said in his interview with Famitsu last July that he picked Robin over Chrom while he was playing Fire Emblem Awakening, basically in pre-production of Smash 4. As for Shulk...well, he was considered one of the single most likely newcomers, and so he was a pretty safe character to guess on.
So the only thing the Chorus Kids have been linked to so far are false leaks. That doesn't discount the chance of a DLC character from Rhythm Heaven, but it won't necessarily be them, and in fact I think Marshal is more likely.
It's not even about one leak or another. The source is the same person. Everyone who has looked deeper into the situation and information came to the conclusion that the person putting this information out is a Bandai Namco employee. Nothing in the leak was a
guess. Leaks are not
guesses. Let's put this into perspective: When Assassin's Creed Unity was leaked, there was a bountiful of information released. So, because the leak didn't have or mention it being heavily multiplayer focused, that doesn't disprove the leak. Leakers only share information they know. And if something is missing in the final product, some times, journalists touch bases on this and the developer would pretty much say the content was either cut or was considered but never implemented. This has happened
many times. Information on games are always being leaked and a majority of the time, there's information leaked out that don't make it into the final product. Not trying to be a smart mouth, but that's game development for you. Things are sliced, diced, cut and thrown out. There's no doubt Chrous Kids were considered at some point just as Chrom was.
The "Pokemon from X/Y" is a huge piece in the April leak because months later in an interview, Greninja was revealed to be referred to "Pokemon from X/Y" internally. Hence why we didn't get a name.
The fear and the freak outs stemmed from people thinking the leak was saying or implying: "This is the FINAL roster! Nothing else!" when that was never the case and hence why the Rosalina argument is moot because, just as I mentioned with Assassin's Creed Unity, there was information and details that appeared in the initial announcement and final product that was never "leaked". It doesn't debunk it.
Leakers, again, just share information they know.