"Which floor is it on": the "it" subject does not refer to the floor (which is the object), but the subject. So I think it doesn't make sense to attribute a "den" (the subject) to the "våning" (the object)
Yes, it has. The word våning is still sometimes used about more luxurious apartments, but normally it only means 'floor' these days ('floor' only as in '1st, 2nd floor', not as in 'the kitchen floor is checkered').
'an apartment' is en lägenhet.