Here might be a possibility.

1. Tell her that you aren't going to play the games. You will let the letter be returned to her. If she would like to mail it back to you regular mail, then you will open it. You will open it only under the following condition: that she ASK you directly the question that is in the letter before you open it. Then you will be allowed a full week to consider your answer. If the question in the letter is different than the question she gives you over the phone, then you will have the option of doing ______ . (Whatever you feel necessary). (Might be sending the letter to her therapist).
2. You will not accept letters from her in the future. They will be unopened whether sent registered mail or regular mail.

As for sending the letters to her therapist, there is the possibility that she worked with her therapist to write the letters.

Sorry