In a filter module plugin, you can edit the content passed to it. But if your edits break other validation rules, the post still goes through. Here's an exaggerated example:
public function filter_question( &$question, &$errors, $oldquestion )
$question['title'] = '';
Now the question title ends up blank and no error is given. It's very reasonable that a question could replace some words and end up at say 19 characters instead of your set minimum of 20.
Is there a way to call Q2A's regular post validation after making changes?