Bear in mind I haven't done nightmare yet so it may not be possible there but...
Have you tried switching the shadow to firebrand? He can simply taunt firebrand during the double destruction, soak the incinerate, let the tank go on tanking firebrand as normal. Voila', no tank swap.
Works PERFECTLY in hard mode, we haven't been doing a tank swap there forever.
If that works, it would make everything much easier.
I thought the same thing. They already have a Guardian dps on firebrand who can do it. Trivializes the incinerate armor mechanic.