If I edit a Bank question directly (in the Question Bank, not the template), does it affect templates that already used it?

 Not unless you choose to. Editing a Bank question that's already in use shows a "Question Used in Templates" conflict modal with two options: Save as new question (keeps your edits separate, leaves existing templates untouched) or Update in all templates (propagates the change everywhere that question is used). Note this only applies to text/options/correct answer — images are always live regardless, as noted above.