I wonder if there's some confusion about what land CMK now owns. It's only the 5 acres immediately south of River City, not the 62 acres south of Roosevelt. There's really no need for any new streets.
I would think there's room for three towers, possibly four, though if I were pondering the market over the next decade I might do two loft-product midrises, and a few years down the road put a good-size tower at the Roosevelt Road end.
I don't know much about P+W as planners. I admire Ralph Johnson's work as an architect quite a bit, but I don't find much reassuring about the site plan and urban design at Skybridge.