I agree with your definitions of the words. My intention of the story is he did not have a clue and he did not understand it. That is the humor there. "Understandingly" wasn't my intention, but it is acceptable, it fits in the story grammatically, although I believe it would remove some of the humor and is scarcely logical. That is why I said both are fine. "I" need you to understand why he did what he did, not how you think he should act.
They have different meanings, but I, who wrote the story, have my own thoughts. Why you want to adjust it to reader’s understanding?
