SBC, it's a tough situation, and I can completely relate. We are not DF'd, so we are still trying to find the right balance with our parents - we want the kids' grandparents to have a role in their lives, we want to maintian our relationships with our parents, and we want them to leave the JW stuff out. Obviously there will be little compromises here and there, and we do our best to innoculate the kids against the madness.
In your situation, giving your mom a little time-out may not be a bad idea. You may find that she's willing to compromise (gasp!) a tiny bit to be able to see her granddaughter. OR...she may view it as opposition and therefore proof that her preaching to your daughter is what Jehovah wants, and what Satan is trying hard to stop.
Sounds like you're doing a great job of teaching her to think. Keep that up, and you have little reason to worry.