Your best bet would be to take a bus to Old Key West (Stop #4), and transfer there to the Downtown Disney bus. Exit at the Pleasure Island stop, and then walk over to Cirque du Soleil.
To get back to WL, just head back to the Pleasure Island bus stop, and hop on the WL bus (ironically, stop #4)
As for time... Murphy's Law says that it could take as much as an hour. If you just missed both buses, and have to wait the full 15 minutes for each, and then 15 minutes travel time for each. Although it is most likely that you will experience a shorter travel time, you should plan to arrive early. I'd be out at the Old Key West bus stop no later than 4:45.
(BTW, although the show is a 6:00 show, it is recommended to be there at 5:30 when seating begins. This allows you to get any refreshments, and find your seat in time for the entertaining pre-show)