Unless the code has changed recently, from what I see the calculation checks for natural skill + item mod values first, and if you don't naturally get the skill, all that is 0. Effective block skill from shields and /shield is calculated and added on top of that value afterward.
Adding an intermediate step to include +Block no matter what would be simple enough, but whether that is something anyone would particularly want to do, I don't know. Probably not even meaningful to wonder about what might have been "intended".
Plus ingame testing has shown that AC has a dramatic effect on avoidance
This is true beyond a certain point. The AC mitigation calculation is not great; I don't think it was ever intended to handle AC values as high as the game now sees.
AC mitigation is highly randomized, hit for hit. One hit you might mitigate 20%, the next 85%. As your AC value rises, the higher the mitigation percentage will be on average, and the higher the maximum potential mitigation will be.
Thing is, the max value on high-end tanks now regularly breaks 100%. When you mitigate by 100% or more, damage is reduced to 0 and counts as a miss, effectively becoming a source of avoidance. As the max value pushes above 100%, the more likely that a mitigation value above 100% will be randomly selected, effectively splitting AC into "AC-mitigation" and "AC-avoidance" chances. Unlike avoidance skills, this AC-avoidance has no cap. Guaranteed passive mitigation from that one AA, overcap STA and what looks like a basic tome line also bump up the max potential value, making it even more likely. Some quick and dirty number crunching seems to show that, for some of the tippy-top tanks right now, avoidance chance from mitigation outshines any single avoidance skill (dodge being the best, as Jumbers said). It also appears to grow at a pretty huge rate (more than 1% absolute chance to avoid) whenever you pass one of the magic thresholds (it's all integer math). It's pretty nutty.
