Good question. For some reason Ars removed a post someone else added (which seemed entirely valid to me), then (re?)locked it. Perhaps you should ask him/her.
Unless the mods and other higher-ups say otherwise, the pool remains closed.
Um... that's... an entirely stupid idea.
Seriously, that is the dumbest explanation I've heard in awhile. Did you even *look* at the pics' pixiv page? IT'S TAGGED AS A PART OF THAT SET. I just looked and the tag is there. It (among others) belongs in the set because it IS obviously a part of that set. And leave it unlocked, since pepepe may add more in the future.