That's easy enough to test on the ops dummy. In fact, people have been doing precisely that since combat logging landed. 10% is a fairly well-verified number at this point.
As for the rest of the explanations, thank you! That clears things up quite a bit.
Not only is it verified on the dummy, it's also confirmed outright by the devs. It was several months back, in fact.
They explicitly said current bosses (and the dummy was built to represent them) have a 10% defense chance.