Is 5th station accessible ?Depends on the road condition, i.e. whether the road is clear of snow or not. Usually in December, it is often still open as far as the 5th Station.
What form of transport is available ?
Rental car, taxi or a guided bus tour from Tokyo, but no buses.
Or should we just visit Lake Kawaguchi ?
That might be more practical. There is not too much to do around the 5th Station anyway.
How cold is it likely to be ? Snow ?
Kawaguchiko average:
Daytime high: 10 degrees
Nighttime low: -2 degrees
Snow depth: 0 cm (Relatively low chance of snow)