Sul should never become possessed. If you DPS him first off the pull, he will be the last of the four in the rotation to become possessed, as Gara'jal will always possess the highest-health Council member that hasn't yet been possessed in the rotation yet.
The objective of the fight is not just to knock Gara'jal out of the possessed Council member; you can wait until about 50 energy to swap and burn. Continue DPSing Sul until he dies or the next Council member reaches 50 energy.
Ok so our comp is boasting a druid and pally tank. Like the OP we too could not persuade the adds to attack the tank regardless of if we stacked or what, and it was GG every time.
So we changed tactics to the one above and had council down in 3 tries (first one was adjusting, 2nd we had people dc and 3rd was the kill).
We did however make a slight adjustment, because of kazra'jin's deflect we decided to wait till 20 before targeting him.
Mal'akk - 50
Kazra'jin - 20
Priestess - 50
Sul was down before the 3rd boss was possessed. We hero'd off the bat along with stormlash (ftw) and nuked the living daylights out of Sul. I think by the time Mal'akk was possessed (always the first) Sul was at around 40% health.
But yup, it worked fine and thanks for the heads up.
To others out there, although the threat mechanic may work as intended, I definitely see there being a problem with Druids and Paladins although I dont know why they have a problem.