You're vastly underrating Koscielny. Your posts read as if you are basing all your assumptions on the few times Koscielny's mistakes have been highlighted in match reports. Koscielny is also carrying a knock this season so he isn't at his best and when he has played, our defence has looked much better. But he was a rock last year, when he played almost every game when we defended very well (around March 2013 to January 2014).
Evans isn't better than him. I saw that ridiculous backpass against Liverpool and in general this season he hasn't impressed. The guy isn't even a regular starter in what is probably the worst United backline in years. If Arsenal hadn't had just six senior defenders this season, there's no way Evans would be anything more than our fourth choice CB. I think Jones and Carrick are far better defensively than Evans. I'd have Jones in our defence in a heartbeat and he'd fit right in too as Wenger likes to have a versatile player in the squad.
Also quite disappointed to see so many people picking mostly United defenders in their combined XI. Gibbs is probably the most underrated defender in the PL. He has been rock solid for us for some time. At the moment, he is better than Shaw. Sure, Shaw might eventually become the better player but Gibbs is the better LB right now. Rafael over Debuchy would probably be closer but I haven't seen much of either player. Debuchy has looked solid whenever he's played for us, but both have been injured for quite some time this season so I am not going to judge them hastily. Mertesacker and Koscielny are a great defensive pair and would definitely make my combined XI along with Gibbs. The forward line is where United have better players so I agree with people picking more United players there. Will probably make my own combined XI later, in a much shorter post.