Not sure if this is the latest presentation, but the plans call for buildout towards the greenway without a basement and then a cantilever. I marked out an estimate of the basement footprint in purple in the second image.2020 presentation
Yes, Fenway is right in the sweet spot for capacity at around 37k. If they added expanded over Landsdown, you'd be looking at mostly empty seat-backs 75% of the time.
I know, it was kind of a bad joke. I wanted to ensure my comment of density on steroids didn't insinuate I was against density. More cowbell....more density!