I can confirm some things concerning bugs for Immerseus. The first thing is that Swirl actually does random damage. There were situations when my raid members didn't have enough time to outrun the Swirl wave and some got killed,some didn't. Basically it did random ticking damage to random raid members.
Just to add on the conversation above - you can't outrange the Swirl wave. It reaches to the end of the platform everytime. Also no defensive spells help other than absorb-like spells.
The bug with Immerseus bugging out if he is killed during casting Swirl is unfortunately active again. We killed him 4-5 times during him casting Swirl and he didn't spawn mobs. He just stood there a few seconds and then started attacking again. The mobs didn't even spawn and he was constantly on 100 corruption.
The black orbs still can't be stunned/rooted. Only slow works. The blue mobs start slowing down only on full hp or near full hp which is really annoying and the later phases get really hard to heal the mobs and also keep the raid up.
The blue/purple pools that spawn just before Swirl are still invisible during Swirl. The only thing I can say is that if you run over them they don't always give you the perma- debuff of 30k ticking damage, but I don't know how and when you get the perma-debuff and when you get hit only once.
The most annoying thing is still Swirl. Even if the amount of damage it does is like on retail (which I seriously doubt) it basically depends on sheer luck to pass the boss. If he starts casting on the raid it is an instant wipe. Considering that you need to go through minimum 3-4 swirls it almost seems as not worth doing the boss and with that the whole instance. I can't think of a boss spell that even if cast directly on you you can't avoid by doing the apropriate mechanic to not get insta-killed.

