Here's the rub -- a sincere, authentic opening up to anyone shouldn't have a hidden agenda. Right now, you are making this all about W, how she will react, what it would mean to how she perceives you, what it would mean about your M, and so on...
You are making it a hugely loaded coversation. If you could talk to her like you might talk to any friend, a cousin, someone here, that would be one thing. But I really don't see that happening. I fear W will feel the pressure to be who you want her to be and respond how you want her to respond and so on...
Any such conversation needs to be ONLY about you and your grandmother. Can you do that?
