Well, it depends on what the story is about. If it's something that is causing harm or is untrue, then I think it's okay for you to take it down. But if it's a positive or important story, I would rather you didn't.
No way. Such an action would be unjustified and could lead to consequences. Stories have their own value and ownership, and removing them without valid cause is not acceptable.
Taking a screenshot of an IG story depends on your device. On most smartphones, you can usually take a screenshot by pressing the power button and volume down button simultaneously. However, Instagram may notify the story creator that you've taken a screenshot, depending on their privacy settings.
Sure. You just press the appropriate buttons on your device while viewing the IG Story. But keep in mind that some users might have settings that prevent screenshots.