I suspect she included the letter because it apparently came with all the poetry when you gave it to her; she's just returning all that you originally sent perhaps thinking the letter was an important frame of reference for you. Just my guess. I don't think she was making any particular point in sending the letter back other than that.

As far as the note in addition to the registered mail: again, probably just a way to be doubly sure that you received the poems back. Maybe she isn't sure she had all of them since it has been so long and this is a way of assuring that she did send back all you were looking for.

You may be trying to find dark, ulterior motives where there may be none. Just send a quick note saying thanks for returning them, and let it be done.