here is how you escape the Citadel. You head to the Flux in the Upper Wards and talk to Captain Anderson at the bottom of the where the cafe is. After you talk to him, he'll ask if you want him to knock out Ambassador Udina or unlock the code to the Normandy through C-Sec. Either way, head to the dock and board the Normandy regularly. And Voila! You're outta there!