I agree that we need a new left back. I don't think Gibbs is as bad as you make out, but with his injury record we need a quality LB, Traore isn't going to cut it.
You seem to have a dislike for Sagna, but I like him. Defensively he is pretty solid, but you are right in that he gets caught upfield a bit too much. However, with the way we play full backs that is going to happen sometimes. The bigger issue is that we don't have midfielders who will naturally drop back and cover. In the past we had Petit, Vieira, Flamini, Gilberto, etc who would cover when required.
I think the CB issue last season wasn't so much the personnel, but more the combination. Arseblog mentioned it recently and it makes sense. Verm and Djourou are more aggressive players, going to meet attackers and when they have the ball they like to take it forward. Kos and Squillaci are the mopping up type of players. When Squillaci and Kos were playing together, Squillaci was asked to take on the more aggressive role which isn't his strength.
The fact that Squillaci's best two games were against Wolves and Everton, where he played the mopper upper behind Djourou, would tend to back this up. Yes, we do need another CB (Jag or Cahill for me, not sure Samba is good enough) but just as important is that we play the right combination of CB's.
When your CB has his best games against a team without a striker, and another team without a striker, then you should worry.
I really dislike this, but they were good in game X so that partnership obviously does work. Song is more than good enough to beat bottom tier premiership teams, that doesn't make him good, just less bad than some of the worst players in the league, thats life, against the top 4 he's laughably bad, he is simply not a good player.
Squillaci and Kos I've never seen with a solid partnership in a solid defence so I can't rule out that with a decent CB alongside them and decent fullbacks, that they might play well.
Sagna is not even close to good defensively, going all the way back to the Liverpool 4-4, and in literally dozens of games since, he has a ridiculously consistant habit of clearing balls from the right side of the box into the middle of the box and then we concede a goal, or a ludicrous pass across the defence which should have been simple but goes to a striker and ends up as a goal. HE gets caught up field not off the back of an attack, but off the back of walking back slowly rather than working hard to get back and set himself.
He's absolutely crap at playing the overlap with a right winger, his passing isn't close to great, his crossing is mostly completely inaccurate, he got a couple of goals but like with Johnson a couple of goals doesn't make up for being crap defensively.
Even when he's in line as said he's full of ridiculous errors and, if ref's ever call him on it, every single time he ends up running after a long ball in behind he flops to the ground with little to no contact, he consistantly wins fouls for it, but also barely 20% of them should be given, most of the time his "play dead" routine should lead to corners/throw ins and people running at goal. His positioning is woeful, the sheer number of times, I remember the Chelsea game where he stands between the left winger to block the ball but he simply doesn't raise a leg, or he leaves for no reason just enough room. Same goes at the other end, I've never seen someone who does two irritating things with crossing, if someone lays on a great ball into space to cross first time, 99% of the time he controls it, slows it down, waits to be closed down then blast it straight into the defender, he also almost never shifts the ball just a foot either way before crossing it, I don't think anyone in the league hits the defender with his crosses as much as he does.
The first 6-7 months at Arsenal, awesome, none of those problems, good defensively, always got back, crossing was better, since he came back from his first injury, he's lazy, stupid, and offers little to the team and frequently the CB's being out of position is actually his fault for being missing.
Djourou is the only CB I'm basically unwilling to give a chance, when we had infinately better CB's around and Djourou got a chance to come on, be a sub or start a game in a solid working defence, he still made mistake after mistake after mistake. He's also injured constantly and you are right about Gibbs, I don't rate him, but even if I did I don't think he's ever been fit for more than half of any season and often a lot less, he's very very slight in build, he gets injured insanely easily and he's just not very good. Tackling wise Clichy is statistically the most successful left back, more tackles and higher success rate than anyone else, Gibbs just doesn't look strong and doesn't look very good in any area. Going forwards he's only got the same drive/workrate as Clichy but little to no end product.