Grandma got the letter followed by the lovely Mother's Day card. She called to thank me for the card, never mentioned the letter. This is her way. We have had several pleasant conversations since pretending that the whole thing never happened. This is Grandma's preferred way of dealing/not dealing with something - just sweep it under the rug and carry on.

Thanks again TE forum sisters for all the suggestions - the input definitely helped me to navigate some very tricky waters.