I guess I'd not plan too far ahead and commit until I arrived in Anchorage. If the weather is crap on the KP, head North first.
My preference would be to skip the Whittier to Valdez trip and spend a day or two in Seward. Even on crappy days, the salmon charters can usually get you some fish in Resurrection Bay. If the forecast looks good, do the Kenai Fjiords Tour.
My experience in booking sight seeing trips for visiting relatives has always gotten us into bad weather.....Whittier to Valdez is a nasty trip if the weather is on the deck. I would just watch the weather and react accordingly.
