Can anyone tell me if there is a hut or other accommodation open at Kawaguchiko 5th station?Yes, there seems to be accommodation around the 5th Station:
http://www.city.fujiyoshida.yamanashi.jp/div/english/html/lodging.html
If so I could travel there Saturday afternoon and start climbing very early Sunday morning so I could get the 15.40 bus back to Kawaguchiko station or a taxi if I am later than this.
It entirely depends on your strength and stamina. The average person requires 8-13 hours for the round trip to the summit.