Is it possible to complete "Pieces of the Past" after killing everyone in Dawnstar?

I am playing through Skyrim on the Xbox 360 and have 49/50 achievements competed. Earlier in the game, to gather a 1000 gold bounty in every city, I went into Dawnstar and slaughtered the first few guards I saw. Without realizing it, my companion killed the leader of the museum.

My last achievement, "Oblivion Walker", was to complete 15 Daedric quests. I completed 14/15, and then saw that I still had "Pieces of the Past". I went to try and start the quest, but was unable to. I searched through Dawnstar, but could not visit the measure any longer.

Do I need to restart my game from a previous save before my companion slaughtered the museum leader? If so, do I need to have all 15 done at the same time, or will I get the achievement upon completion of that quest?

You can't start the quest if you've killed Silus prematurely, you'll have to revert to an older save. There was (maybe still is) a bug with Hircine quest, allowing you to acquire 1 extra artifacts, but that's not an option if you've already completed it. You'll have to reload. You need to have completed all of the quest on your current save to unlock the achievement.