The lake is big, but not that big. The island can be backed with grandstands facing out. Furthermore, all of the course, minus the apartments and onramp, is flanked by praking lots, big flat spaces to put grandstands. If 670e is also closed, Grandstands can be put there (makes sense if the pits and finish will be there). I'd say that over 100% would be visible from some seating, some where. Thats more than you can say for some tracks.
Runoff space is a legitimate concern. This isn't going to be a 'plug-n-play' event, the city will have to invest in a few changes: Removable center dividers on the highway, temp catch-barriers pretty much everywhere, safer barriers and sand traps, or possibly some of the more modern speed-scrubbing surfaces. Walled tracks with few places to run-off aren't unheard of.
There is also plenty of room for stewards. Keep in mind, a nest isn't that big and we wont be using all of the road surface.