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.
This does work on 8man nightmare. Can't confirm on 16man.
Also if you do this you prob want your vanguard on firebrand, as he's all mitigatable, vs storm caller who eats right thru VG/PT defenses. Guardians have better mitigation vs storm caller's elemental damage and cooldowns that work on that damage.