Actually, Shield oath and Defiance are very very close to each other (yes, pld has a slight advantage, but I'm about to explain why is quite minor).
Say a pld has 6,000 hp. The equally geared warrior has 7500 hp.
Twintania uses Deathsentence and hits for 4000.
The pld will take 3200 damage. 53% of their max hp.
The warrior takes 4000 damage. 53% of their max hp.
Scholar uses physic for 1000.
Paladin regains 1000 hp. 16.6% of total hp. They are now at 69.9% cap hp. (4176)
Warrior regains 1200 hp. 16% of hp. They are now at 69% cap hp. (5175)
At this point, twin needs to deal 5220 damage to kill the paladin, or 5175 to kill the warrior (this is a less than 1% difference in the two classes.)
As you can see, using numbers that, while rounded, are actually rather accurate to current raid conditions, defiance and shield oath are nearly equal. Shield oath has a very minor advantage (0.8%).
You are exactly right that inner beast compares complexly with rampart. That said, I do think IB will, in most raid situations, be superior.
Because of IB's very short cool down, you get to use it for every single major attack in all current encounters (all upheavals, all death sentences, even all triumvirates if you really wanted to play that game). Rampart, by comparison, is at best every other, or sometimes even every third. You could even have IB for every turn tanking ADS (since the most dangerous moments in the fight for tanks are during the transition, this is quite potent). Also, in all of these scenarios, there is no reason to think the warrior would have any problem building rage stacks. (No kiting, no dps interruption). And, since you are never more than 20 seconds away from your next IB, you can just spam it, meaning you'll get closer to 100% theory uptime where as on most of these encounters rampart needs to be held (Meaning you don't actually get its 100% theory up time).