That was accounted. Bear in mind that PLD also gets greater mileage out of its cooldowns: Rampart hits 1/3 of bursts where it covers 2/9 of total time, Sentinel gets 1/6 bursts where it covers 1/18 of total time, etc. You are also comparing an autoattack-mitigation picture which includes all flat self-healing already. Without that flat healing, WAR would actually be a bit behind PLD -- WAR simply has weaker scaled mitigation over time. In the additional burst scenarios, that healing has already been counted. You can only count the scaled effects on those additional burst attacks, and because all of it is scaled, the exact amount of damage is not relevant; the burst reduction ratio will be true for all 30s-interval attacks within a 15-minute window. Even with that sort of burst schedule, the two just aren't seeing much difference in total mitigation. Another way of looking at the problem is that while WAR sees greater benefit than normal from the attacks, it still effectively moves you upwards in total damage rate -- the additional utility just help close the additional gap that would have otherwise emerged.
That said, I will generally agree with your assessment. I believe that WAR will be preferred for Twintania and Titan not because of total mitigation but because of eHP. PLD isn't technically harder on healers' MP pool, but WAR would give the party more breathing room in long encounters with reliable burst mechanics. PLD can make up some of the difference with Stoneskin, but it's simply not going to compare to WAR's superior eHP extension.
Not yet. Shouldn't be too complicated though, so I'll do it now. Old Inner Beast can push 1600 healed per shot at the top end (ilvl90 gear and strength build). To be equal between the buffed and the current versions, you would need to reduce out 1067 damage. That means the enemy would need to deliver 5333 damage over 6 seconds, which many opponents will do pretty reliably. My low-heal condition for this and my previous thread were 1133 healed from Inner Beast, or DL+Garuda setup. That would be an equivalent of 3778 over 6 seconds, which is less than many opponents deliver in a single hit, and if you were to remove the occasionally-dodgy inclusion of Storm's Eye, it's only 3400 for equivalence, or 567 incoming DPS. So is it a buff? Well, yes, but a small one. So yeah, the change isn't too much of a buff on the mitigation side, but it does reverse the gear scaling problem.
As I said in my previous balance thread, though, Inner Beast was never really the problem so much as Wrath and eHP. My guess is that the dev team decided that instead of giving a large passive HP to take advantage of Inner Beast, they would rather make the ability an eHP extension.
//EDIT: Maybe it would be better to think of it this way. Previously, you could drop 5500 in healing per minute from IB, roughly. Now, you'd get back 1833 from healing, plus 18 DPS-seconds at 500 DPS for 2250, plus 20% of two Death Sentences -- 1000-1200 each. That would put you at 6100-6500 or so neglecting the impact of other abilities, which comparably reduce the value of your defenses. Vengeance, for example, would take out 30% one way or the other, so you'd get 30% less from that one IB drop. Considering that we're talking 300-360 from Death Sentence and another 180 or so from autoattacks, you're looking at the difference right there. Thing is, you now get eHP out of it, and that's really the key to the whole bit. The big problems with Inner Beast before were that you were dead before you could use it, and gimped once you did.