Correct. Grenadier Pond used to be a bay off Lake Ontario, until silt from the Scarborough Bluffs deposited a sandbar in front of it and several other ponds (formerly bays) around it. The city and railways made the sandbar bigger to handle 2 tracks, and later 4 tracks. The city then added more landfill to create Sunnyside Amusement Park, before it was demolished for the Gardiner Expressway.The others, assuming I can muster an intelligent analysis, will have to wait; the last one is non-starter.
You're right on top of Toronto's brand new sewage outfall, and its venting system. For that reason alone it would be a no. You're also intercepting sand that moves from the bluffs area to the Spit currently, which may have erosion issues that result.