Well, I have to say, as someone who lives in Anchorage and who has sailed in and out of Whittier, I think you are correct with the available options. I wish I had better news for you. The Avis in Whittier is always extremely expensive and sells out early, but if you want to go straight to Seward, that would be your option, so book early.
This would not save you any time, but it could be viewed as a fun idea. You could take the Princess transfer to Anchorage. Spend the day and night in Anchorage exploring. Then the next morning, take the train to Seward for the day, or even for one night, and take the train back to Anchorage. The train to Seward goes through a valley that is inaccessible by cars and it’s beautiful. The train ride does take longer than driving there, but it’s so relaxing and beautiful.