I'm not very familiar with the changes Requiem makes (except you can't really use skills unless you invest perks in them), but like others have said, Windhelm is a fairly good place for stealth kills. Especially if you have a lighting mod and/or ENB that makes it darker.
Have you by any chance taken perks in Alchemy? I've noticed Requiem makes potions, and more importantly poisons pretty darn strong. One sneak attack with a poisoned arrow could be all that's needed.
Breaking and entering most likely requires lockpicking, which in turn requires perks. I believe the Shatter-Shield residence has an Expert lock, so... you'd have to
be an expert and invested in the skill.
In any case, good luck.