A weakaura for glimpse soaking would be nice. Something like: You're taking damage from it (and therefore are the closest). You have X stacks. When you get Y stacks, swap. Basically, something to play the game for me.
Bigwigs/DBM might do something like that but I didn't notice anything when I was near one of the shadows. (I think I was the closest. I don't have logs to verify...)
Phase 3:
While we noted that it would be difficult to get the Branded onto specific individuals, I think we can do more to control it. If you get it, move towards another individual so as to control the bouncing to be between the two of you. If you see someone near you get it, move towards them for the same reason. (obviously communicate if multiple people move towards the same person) Yes, you need to be careful when force novas come out, but if the person who gets the 7th jump of branded knows they are going to get it and are prepared to leave the group, they are more likely to successfully remove it.
^ This. Was consistently taking it off Boggy/others near me at 6-7 stacks so I could just easily run it out with deterrence. Seemed to help.
^ Sarcasm
Would a third tank to help pick up and manage adds help with the transition to phase 4? We can probably afford the DPS loss.
Yeah I've been considering whether to try 3 tanks or not, as I know it was a common strategy for early kills and such. I'd love to be able to take on a third tank role myself but I think I'm too well suited (and not practiced) to handling Arcane Wrath at this point.
Still, an add tank that can handle Arcane adds during Phases 1-3 and then pickup half the adds (from either east or west balcony) during Phase 4 would certainly simplify the transition periods as Boggy mentions. Neph as a DK offtank would likely work well due to AoE DPS, self-healing, and grip, though I suppose most any class could do it.