I do not know if it was a corner to start with. As for the penalty, you'll find Kolcielny was penalised earlier in the season for a bit of mutual grabbing with an attacker in the penalty area for a set-piece. A penalty was given against us. It was in the rule book but not often given by referees. So, there you are.
Regarding Walcott being offside. Yes, he was. But, the linesman was behind play. So, he could not possibly give that decision.