Are spouses considered divorced upon the written utterance of a divorce oath, and what is the ruling on the marital relationship thereafter, and what must the husband do to rectify the matter?

If the husband did not intend divorce when writing the message for you to go to your grandfather's house, then no divorce occurs. Similarly, if he did intend it, but then permitted you to go based on his belief that his oath would not be binding, the preponderant opinion is that no divorce occurs either. The determinant factor is the intention. So, if the reason is removed, or there was forgetfulness or an interpretation (ta'wil), then no divorce occurs. Therefore, you are still under the marital bond with your husband.