It is a pretty buggy quest, even after the updates. If you left the city with Ulfric, the Civil War quests won't affect it. There are some pre-reqs to start the quest, which include visiting the city at least 4 times and these following conditions.
[*]Enter Windhelm through the main gate, docks, or fast traveling between 7pm - 7am. This triggers the graveyard scene.
[*]Tova Shatter-Shield and Arivanya need to be alive. If Hjerim's lock is "key required", you haven't met these requirements. Otherwise the lock will be a normal master lock. I'm just mentioning all this because it jumped to my mind when you said you had to steal the key to the house. Viola also isn't a part of the quest until you've done several steps, so it's not alarming she won't respond yet. After the murder scene (which you may or may not have seen, but check the way I mentioned above), Jorleif starts the quest.