The ref got both decisions spot on. Alonso got the ball before man and Koscielny was the last man. Arsenal are the perenial losers.
Is it not a yellow if there was an attempt to get the ball under the new rules? Not sure he did mind you, although I thought it was quite a dive.
I agree if he had attempted to make a tackle, but when it is a push I still think the ref is in his right to award the red. Believe there is something in the rules about "making a genuine attempt" at winning the ball. But I'm not 100% sure so will stand corrected if someone can clarify.