For the 1st part when you need to execute healers I found it best to Mind Sear since in our tactic there are 3 healers.
After healers(or whoever is your first pack, but let's assume you're using common strategy) you may be a bit soaked on mana, that's where
a)using Mindbender on Mel'Jarak with his 1 stack of Reckless
b)placing VT on him(FDLC)
I'm usually choosing A, to not to switch to WL for every FDLC proc. But
If you would like to mind sear till the very end, you might take ToF.
If you would like to mind sear a bit less, you are taking DI.
I am taking DI. So far the funny reason is-I'm placing 3 SW:P and then Mindsearing through boss again, and spend of MB without switching from boss in target(which equals to more DP in my raid's DPS perspective).It may be not as effective as ToF+new Mind Sear, but I just still used to place not so many Mind Searing, pre 5.1 tradition, and doing more and more nuking damage to single target. Still, for 10ppl normal everything is fine, except multidot for now(we may deal competitive damage now).
I know people who Mindsearing for two first packs, I know ones who are mindsearing 1st pack and then start multidotting Boss and 2 cleavers with DI(sometimes FDLC, the reason you ignore it is casting time while mindbender may deal more damage on reckless boss then even all FDLC procs, less target swiching=more stable rotation, except DI procs when you may get 3 procs for 5 seconds and place DP on the boss straight forward).
TBH, with new Mind sear WL Mel'Jarak is just amazing boss to fight against as a Shadow Priest, you may test when it is more suitable to add some multidot to your Mindsearing on Phace one, that really makes lots of fun.
For so fast target switching between boss(to spend all DI:FDLC procs or Mindbender) and his adds I'm using TidyPlates, Need to Know and really cool Boss Frame from Shadowed Unit Frames. I'm not using focus on boss at all because Focus is binded to my Sapping Spear(or how it named in English lol, you got the point) by macro.
Make sure if you do use Tidyplates you check the option to make nameplates not overlap with each other. That's pretty damn important for easily switching between them, otherwise it looks like one really large blurry nameplate.
Halo is about twice the damage/healing per target (maybe more like 2.5x damage) every 40 seconds instead of every 15 seconds. If you hit DS on cooldown you'll get similar or possibly better results than halo. It's much less position dependent too, and has a much better self-heal.
Halo gives a bigger single punch though and if all the targets aren't stacked then it can be a lot better.
I will admit that it's rather easy to be in optimal Halo range on this fight. I'd only take Divine Star if your guild's strat is such that hitting the AoE pack using Halo without hitting the CCed mobs is (close to) impossible.
I've started using Divine star a lot more after the buff. It is now better on all fights where positioning makes halo hard to use and its also a tad easier to use. But last night I ranked #3 with divine star on Windlord so I think it might be better there too.
hmm. The biggest issue I see with Halo is the same one that snaxattax finds with Divine Star. Not all the adds are grouped up on exactly the same spot, and even 5 yards separation is enough to kill the effectiveness of halo on some of the adds.