One of the comments from the video said that mission on Citadel should be started BEFORE attacking Illusive Man's HQ and you only do it AFTER the Cerberus/Udina coup attempt on it.
So far, my savegame replays are currently AFTER the Udina/Ceberus takeover Citadel and I'm still doing Liara's investigation of the missing Asari Colony before the war on Thessia.. Should be interesting if I played this DLC before it happens..
Thane and Mordin dies yes, but you'll get to see KOLYAT, Thane's son based on that trailer... I hope Legion shows up in this DLC BEFORE he dies doing his mission to rewrite the geth with Tali via Reaper Code uploading stuff.
