Disclaimer: this is sort of analysis is necessarily rough--beyond these basic estimates, talents and many other things will affect mitigation and overall tank performance in any given context.
Disclaimer 2: I didn't review your calculations or assumptions, but the approach seems reasonable.
Even based on your own conclusions, you're greatly downplaying the Warrior
's mitigation advantage in your example. "Only 6.64%" is a very large difference when the Druid
is only taking 33.57% of incoming damage in the first place. The Warrior (who's taking 26.93% of incoming damage) is taking 19.8% less damage than a Druid
. 19.8% is a very large mitigation advantage--it amounts to a 25% increase in effective health, which is very close to the 29% higher health the Druid
has in your example.