^ Actually, "kabe" is the wall, while "don" is the slamming sound being made. Oppaidon would only denote boobs slamming something, in this case, another set of smaller boobs. This particular picture shows both kabedon and oppaidon.
Rena was definitely the first impression I got, though I expected to be wrong when I opened the image. Not sure what to make of Iroha pushing Rena down, anyway(except Kaede's murder switch being flipped, of course).