Would my wife be divorced if I said to her—with the intention of admonishment and discipline, not divorce—"Consider that I have sworn an oath upon you"? And if she is divorced, how can I take her back?

This phrase is not an explicit divorce pronouncement. As long as you did not intend divorce by it, no divorce has occurred. Our advice to you and your wife is to live together righteously, and if a disagreement arises between you, avoid threatening divorce, whether explicitly or implicitly.