Is it safe to freeze breastmilk and give it to your baby months later?

Dump it. Freezing alters the breast milk enough that it has little or no benefit if it is more than a few days to a week or so old. The antibody factors are ruined and the material is not as good as basic formula at that point.
Probably not. I am not a pediatrician, but I do know that freezing causes water to be nasty micro-spears to other molecules. Contact your local la leche society-your OB will know. These ladies are smart breast milk sharers. So you will have a safe supply of breast milk, nature's perfect food.