2 things, as Visual defended me (thank you), I was not assuming that all DKs are Unholy, my goal was simply to offer a scope of comparison. Compare an Unholy DK with a 2-hander to a Frost DK dual wielding allows me to compare my current spec (and not an uncommon one) with the proposed "best" dual wield spec. My "napkin math" was simply to show that there was a scenario in which wielding a 2-hander wasn't so different than the dual wielding in another form, since you will not actually use the same rotation, abilities, or spec for changing weapons. It's nice to set those as statics in the equation, but not accurate. Point of fact many 2-hand Frost tanks will favor Oblit because it is better threat (better single target damage with a good epic 2-hander, and less chance to fail to land even with parry chance) and they will not avoid it because of the parry liability. Similarly other tanks were put in to provide a bit of equivalency, however cursory to set the stage for DK parry liability next to other tanks.
There is an uncertainty on Rune Strike that will have a major impact on your math as well, and it is one we haven't demonstrated yet. Namely, will Rune Strike replace an off-hand swing? If it does the dual wield DK will be able to make much better use of their Rune Strikes. Your math appears sound on opportunity for RS usage, but the numbers don't ring true in calculation. You're suggesting that with a weapon that is 30% faster, you'll only be able to use 10% more RS's, when really the thing that will be obstrusive with the 2-hander is the awkward windows have having a swing timer that will be so out of phase with a faster boss swing.
On top of that, neither of our math solutions take into account the interesting compounding effect of our being avoidance tanks. We are avoidance driven and the more our gear improves the more value we get from Rune Strike, and the more parry haste becomes a reduced liability. Bear with me through some numberless logic:
Boss swings, we dodge. At this point we've avoided one attack and reduced our vulnerability on the next melee swing since it cannot be dodged or parried, which means better hit chance and no parry liability. Now, we swing, we are parried, the boss gets to squeeze more swings into the same span of time. However, the boss's extra swings are ALSO still avoidable. More swings from the boss actually increases our opportunity to avoid and proc additional RS's. So, in a way there is a decay. Rather than parries simply increasing how much we get hit, it increases our opportunity to use Rune Strike, not just our ability to use it more, and it doesn't increase the damage flatly. You say 30% more parriable swings, but I flavor that with of the swings that are parried, 15% say, if we had zero expertise, 15% of 30% means 4.5% more parries, and if we still have 50% avoidance we're further cutting the fallout in half.
In fact, dual wielding actually has a self-compounding protection factor to compensate for the increase in parry liability by reducing parry liability and simultaneously buffing threat output.