Tarrondor

Depending on the iLvl difference, I will. ( sims for single/three/six targets respectively )

Bloodthirsty.

You're going to want to go with the 900 iLvl relic over the 880 iLvl relic. That's 20 iLvls to your weapon. EXTREMELY important to have that as high a level as you can get it. The only time you need to concern yourself with the actual traits that the relic is affecting is when it is between 1-4 iLvls.

It's really going to depend on the situation. MoC for pure single target. Barrage you'll be using in many of the raid encounters as the mobs will be more spread rendering Volley not as effective. Volley will be used for Mythic+ dungeons since gathering and grouping mobs will be a leading tactic. For leveling, go for whatever "shoots" your fancy. I don't like Barrage for leveling as I may pull more unintended mobs, so I bounce between MoC and Volley.