When I travel to the other end of Lake Ontario and view my home city of Toronto from the city of St. Catharines, why does the CN Tower appear 2/3 as tall as it should?
If the Earth was flat the CN Tower should appear as-is. In the image below, you can see a Mirage Effect (eg. where the white roofed Rogers Center is smeared) but due to its gigantic height you can see how low the CN Tower dips below the horizon.

Your superimposed image shows buildings that are in your original image therefore they have not been 'hidden' by any such curve. And as you point out the white building has taken on a mirage effect. Any reason the white building features twice (above and/or below itself) in the superimposed pic?