Yes you can walk it. We did it the day before without luggage, just to see the beautiful Golden Necklace Park And beautiful harbor. We chose to take a taxi to the ship the next day just because of our age. I don't remember it being expensive---around $10. We came from Narita airport using the same limo bus service you mentioned. No buses go to the port. On advice of the bus desk (show them your hotel address) we rode to the Yokohama air Terminal which is really the bus station and took a taxi from there to the hotel. Once again, don't remember it being expensive but we rode through a most breathtaking shopping area of high rises.

I never took taxi from hotel to Haneada air port. It would cost pretty expensive. I suggest you that take taxi to Yokohama bus terminals then take air port shuttles from there to Haneda. Its runs every 10min and its only 25 minutes ride. I think cost is around 1000 yen. I hope this will help your trip plan.