Y'all are making this much more complicated than it needs to be. The simplest solution is to extend the or to 95th Street to provide additional service. I would advocate for the because extending it would mean that Williamsburg is a simple cross platform transfer away from all of the BMT Southern Division lines. Creating a stub line in this case would be simply wasteful.
I read at some point that 95th Street is restricted to turning 10 trains per hour. Since the is scheduled at that, any additional service would require some kind of change at the terminal, assuming the restriction is true. It could be false information for all I know, though.