I completely agree that if you can, you should leave MOSTLY fresh, with only frozen as a backup. Freezing kills some of the good qualities in the milk, as well as making it less portable. I would give it to her in the morning when you drop off DD, and if someone else does the drop off, send the milk in a small cooler with them. Make it part of the routine. When your mom sees that it's a lot easier to warm up and deal with during the day, she might change her mind. I would pump straight into a bottle, so all you had to do was screw on a nipple top, set it in a yogurt container of hot tap water, wait a few minutes, feed. Lots easier for everyone involved, really. And healthier!