It's not possible to have only one squadmate survive and have Shepard survive as well. This is for gameplay mechanics. Since you can still do missions after the suicide mission, you need to have two squadmates to fill out a full squad. Even though you normally have the option to leave Zaeed to die if you take the paragon option with his loyalty mission, you will not be given this option after the suicide mission if he is your second surviving squadmate.
This is kind of like how it was possible to keep Wrex alive during the Virmire confrontation even if you hadn't done his family armor quest and did not have the necessary charm/intimidate points. If you didn't have Garrus or Liara at the time of the mission, Wrex lives by default because he's needed along with Tali to fill out your three-man squad when you don't have Ashley or Kaidan as an option.
ETA: Also, if you think you've managed to have only one squadmate survive, check your team again after the suicide mission is done. You'll discover that one has miraculously survived the Hold the Line defense.
Modifié par Pacifien, 06 juillet 2010 - 02:14 .