Mortal Strike in PvE

One of the people in our raid core this expansion is adamant that Blizzard designs 10-man fights to use at most 2 tanks and at most 3 healers. He gets really mad when we try to do things with 3 tanks or 4 healers because that's "not intended" and there has to be a normal way to get things done. I don't have any faith that Blizzard knows a thing about balancing 10-man raids (Sarth+3 anyone?) so especially right at the start of a new tier of content I think trying lots of things makes sense even if it's "not intended".

We pulled our first hard mode of the expansion last night and I made the comment that I didn't think 2 tanks was even remotely feasible due to the fight mechanics, which got another snippy comment about Blizzard's intentions. So, it's time to justify to myself with math why I don't think 2 tanks is feasible from a numbers point of view let alone a logistical point of view. The primary reason I feel this way is a healing taken debuff applied to one of the tanks, so we need to assess what this debuff essentially does.

If a tank isn't taking very much damage (Kurinaxx, 2nd boss in BM, or the dogs in ICC as examples) or the healers can trivially overheal them then a mortal strike debuff doesn't really do anything. It turns the abundance of overhealing into actual healing and reduces HPS numbers across the board without actually impacting how the fight works out. The healers just do their thing and everything is fine. The only real concern here is if the buff ever stacks to 100% healing reduction, because then the tank is just going to die.

If the damage is high enough that the tank is actually in danger of dying normally from spikes (Broodlord in BWL) or if healer mana or throughput is a real concern then things are different. In the spiking out case the tank's 'time to full' goes up which makes it more likely he gets exploded, but if you have enough extra healing you can just brute force this out.

In the mana or throughput case you can model the mortal strike debuff as proportionally increasing the tank's maximum health pool and incoming damage taken. (A lot like comparing a stamina stacking tank to a block stacking tank, actually.) For a while this doesn't hurt because the healers won't have any trouble keeping the tank alive, but eventually the amount of 'damage' they have to heal gets absurdly high.

The time for the tank to go from full to dead remains the same since both their maximum health and their incoming damage remain proportionally the same, but the amount of incoming damage just keeps going up and up until the healers either run out of mana or simply become unable to deal with the amount of damage. So if we know how much the healers can handle we can work out how high a mortal strike proportion we can live with. To know these things we have to know how much damage the fight is putting out and what the healers can deal with both short term and long term.

The fight I'm interested in is heroic Halfus in Bastion of Twilight. First let's look at the absurd case of a single tank and see where we become unsustainable, but to do that we need to take a look at how much damage is being done by each of the parts of the fight. I tanked each part of the fight that we released last night, so all numbers used here will be for me (a stamina tank, not a block tank).