The meaning of a legal ruling

peace.
I wanted to ask, does a rabbi, when he rules over a halakha, create it, or does he merely discover it?
Specifically, a divorced woman wants to live with a priest, and is trying to annul her first marriage on the grounds that the marriage was forced by the family. She will live with that priest anyway, whether with permission or with prohibition, but if possible she prefers with permission (she is not religious).
Is it being said that it is advisable for her to annul the previous marriage so that she will be considered free, or is it being said that since she will be living with the other person anyway, if it is known to Shmaya that the first marriage was forced, then she is not now committing any prohibition, and if it is known to Shmaya that the first marriage is valid, it will not be helpful even if the rabbi annuls it?
I remember that you discussed this topic in the past. I would be happy if, in addition to answering, you would also give me a link or reference to your articles and those of others on the subject.