As far as I am aware, freezing does not make fats harmful to eat. (Plenty of people here freeze large pieces of meat[protein and fat] for later consumption without issues. Of coarse, fresh is usually preferred for taste reasons)
As for the grainy consistency, I dont know a lot about this: but I imagine that after freezing, the milk "separated." Milk is an emulsion of fat and water. That is why it is opaque instead of transparent. When you freeze it, different parts become solid at different temps and the emulsion "separates." Then when you bring it back up to temperature, the fat and water remain separated and grainy. To reform some of the emulsion, you could mix/shake/blenderize it. It should regain some of its normal consistency.