Only 15 dead? You have a problem, not enough dead bees!
jep. I would say that 750 is normal figure.
let the bees be in peace. When snow is melted, then it is good to lift the hive off and clean the whole bottom.
Only what you may do is to weight with hand, is it heavy enough. If if feels light, open inner cover and look, how much you see capped food in upper parts frames.