I think you've just shown why the original "rule" you heard, is wrong.
The only one of the bolded examples that even bears a second glance is "now Bill was really knowing what he was saying." I think JK Rowling here chose this word (instead of a more common Bill really knew) to show the currency of the event. To say he knew, might suggest it was an older knowledge or understanding. Now, however, he is really knowing, really understanding.