If it's store bought, I'd say there's a 99% chance it's been pasteurized. Here in Georgia even the apple orchards flash pasteurize the juice before selling it. I've always assumed it was a state or federal law. (but I would be wrong)
Even it it's not pasteurized, if it's in a shelf-stable, sealed container, it's probably free of wild bugs. Otherwise it would ferment in the container.
Lactose will add body to your cider and some residual sweetness, but I wouldn't count on it back sweetening to high gravity levels. If you are looking for a non-fermentable sweetener, stevia or an artificial type will be your best bet.