I have to disagree with you. The thing Kos does is run at the ball. He is good at attacking the ball but positionally and tactically he is a bit naive. A lot of the goals we have conceded have been when he has attacked the ball when (in particular) TV was already attacking it. He should have been staying back and covering other players who went on to score. On the other hand Mert (being slower) is great at positional and tactical play (which is why he gets in the German team). My opinion is that its the combination of the 2 styles that works best which is why Kos and TV does not work as theyre too similar, but TV has better alround awareness. When those 2 play together we've had loads of horrendous mix ups. As for the rest, they are all international defenders (even Jenkinson has been called up for England). There have been rumours that Bould is not happy with Wengers training and that his insistance that they maintain as much attacking philosophy as possible that it is leaving them defensively lacking. So its not down to the players but the tactics and Wenger, according to the rumours we hear at least. There is evidence to support this as well such as Arteta playing as the holding CM rather than a DM.
I disagree with you too. We have absolutely the best away defence in the league. Our problem this year has largely been at home when we are chasing a game. This is when we take risks pushing forward. That is just part of the game, but berating the defense because the attack can't score with 68% possession for an hour is really unfair. Aston Villa was the last straw for me. the defense were absolutely superb, gave up a goal on a speculatve long shot, after giving them maybe 3 chances all game, and then got ripped for poor defending! They might be the best in the league and are far better than the Manchester United, Spurs, or Chelsea defences. Man City's might be better, but they have a far more conservative approach to games. You have to appreciate that ANY defense give up chances, and most of them give up a hell of a lot more than ours do.