If you spend 2 weeks in Niseko in Jan or Feb (lucky bastage) you will be pretty safe in terms of powder.
You can get bread, yoghurt, fruit, bacon, eggs etc at the either of the 2 convenience stores.
As for the train, definitely agree hhaidar that the train is to be avoided if possible. The scenery is nice and the Airport to Otaru express leg which passes by the coast is fine (discovered that Hokkaido has good surf too !), but the Otaru to Kutchan leg on the local train is a chore. I took the train on my last trip in the beginning of April when the bus stopped running regularly and it is an experience I would prefer not to repeat.