I think that I once heard someone officially say that armor and critical damage probability are independent in the game. The person sounded very knowledgeable, although I really do wish that I had something more solid to cite with. It's entirely possible that a patch may have changed that. Still, I think that unless a dev says otherwise or someone else has some solid information to the contrary, we can safely assume that they are independent.

Perhaps there is an upper limit to crit. What we know for sure is that the upper limit can't be more than 100% - ex: every shot lands a crit, which effectively doubles your damage.

Also, are special abilities affected by crit the same way that weapons damage is? It could be that special abilities like blast shot described earlier are less likely to hit a crit. I suppose though again, we should assume until proven otherwise that critical hits work the same way. If that is the case, focus is the ability to cast first for birds as is blessings of might for mages.