1) My group tanks the spirit/boss on the left side (facing Gorefiend) and keeps the corrupted souls on the right side of the boss. This also prevents chimera shots directed at the spirit from accidently spreading to the souls.
2) You just have a lot of haste to the point that warsong procs are pushing you over the 50% soft cap. The devaluation on your haste scale factors does not mean that trading haste for multistrike would be a net positive. A reforge plot at 25 point steps (so 3 steps = 1 gem) shows that changing a single gem would be a loss. Multistrike is typically better than mastery, so that's no surprise. Furthermore, when I ran your stat weights it was .55 for multi, .50 for the rest. That's of minuscule value. Other than getting Tyrant legs out of your cache or Zakuun chest + Mythic Tier Legs from Gorefiend, no gear will be an upgrade for you other than item level bonuses of your current stuff.