You can cold thaw the bag then refridgerate for 24 hours - pop the bag in cold water and it should take about 20/30 mins to defrost. If you defrost using lukewarm or hot water then I wouldn’t recommend to use it again later.
As long as the milk is cold it should be fine
I would get a bag in the fridge so it slowly defrosted. Things that slowly defrost tend to last longer than quick defrosting