ICC buffs and debuffs not considered?
Regardless, you fall into the usual pitfall. A trinket on a random or semi-random proc isn't reliable to be active when you most need it. This trinket does a halfway decent job, but I still don't really like the proc. It can still be leapfrogged, and it may not do a damn thing. (Once you get hit to under 35%, your biggest worry is that next hit - avoidance does nothing reliable there.)
In your assumptions, you have a boss fight with a boss swinging at 1 second speed, hitting you to under 35% every blow. That boss will kill you, and this trinket will not make a difference.
I don't know where you get your avoidance values from, but I'm not sure if this is talking as a paladin or warrior. A warrior shouldn't be able to have 30% block chance in reasonable gear, and a paladin doesn't have shield block. You also forgot to add the chance to be missed to your avoidance numbers, it's not just parry and dodge.
For some reason you seem to think that unhittable means that a boss can't hit you. This is not the case. Unhittable means there will be no normal hits on the combat table. Blocks are still hits, and are definatly not within the 9.7% chance to be hit you used. (Again, where did this number come from?)