User blog comment:LilyheartsLightning/Citadel and Reckoning DLC announced/@comment-5258512-20130224040227/@comment-24174486-20130224041513

Corey Gaspur implied on Twitter that the Lancer might appear in single-player; it's fairly likely that there'll be more new weapons in Citadel.

I don't see any reason as to why dead squadmates would prevent you from playing the Citadel content. I imagine that various scenes will differ significantly depending on who's alive and who isn't.