I believe that Rumple couldn’t get near to Regina, as Cora wasn’t having it. And so after Cora smothered Regina, she turned to him for help and shoved her mother through the mirror. So Rumple had to have written that letter to Cora after she was sent to WL….
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"If you go as far as you can see...you will then see enough to go even further." - Finn Balor