Sorry you had all of this going on, but really the best person to ask is the consultant who has given you this treatment- you need to ask all these questions and have a full discussion.
I don't know why you would be offered progestogens (Provera or the Mirena coil) for a precancerous polyp- unless we assume the polyp was in your womb?
The treatment for any womb cancer or pre cancer is what you have had. It blocks the effects of any estrogen in your body which would cause the cancer to grow, so they are being careful even though the polyp was removed.
I don't quite get why the dr agreed for you to stop the provera - surely they told you why?
When anyone stops using progestogens, they get bleeding if there is a lining in the womb to be shed. This in your case, is to make sure it doesn't turn cancerous, as you are post meno- it's a form of clearing it all out.
I am sorry but I don't understand why this is not being explained, and why they have not told you how long you need treatment. You must need this treatment to help.
It's for your consultant to decide and explain to you. See your GP and ask him all of this.