The Saikyo line southwards becomes the Rinkai line and it runs at least every 10 minutes, or more often. It is easy to take to Shinagawa Seaside station. Also it skips a number of the Yamanote line stops so it's faster too, worth the few minutes extra wait.

And the way you described back to Haneda works fine, I've done it. You have to walk about 200m to transfer. There are escalators to ground level on the Rinkai line and a lift to the monorail, hopefully all are turned on currently.