So I dove back in, and here's a summary of the story so far:
A murky contact of a Brawl modding group supplied them with images from an ESRB DVD submission for Smash 4 3DS. The members of the group swore not to share the photos, but after a week someone had broken the covenant and a party, not one of the original circle, made it known that he would leak a few of the images. Some faction of the original group tried to beat the rogue leaker to the punch, planning to alter the images and leak them first to cast doubt on the whole thing when discrepancies and photoshop artifacts were noticed. But they weren't fast enough, and Leaker 1 happened. Leaker 2 and 3 then posted in response (the original 4Chan posts are quite antagonistic).
On Smashboards, someone claiming to be the first leaker has now said that he posted the DLC list as a trap to identify fakers who tried to pile on and claim they were in the know. The mods at Smashboards have "verified" the identity of this person, but that means absolutely nothing.
I feel like there's going to be a murder involved in a week or two, which will not shed light on the truth of the leak.