The Bürkliplatz is named after a guy called Bürkli who was cheifly responsible for reclaiming that area. The lake shores were rather marshy in those days, and as you say, the town stopped well short of the present waterfront. As part of a bigger scheme to regulate the water level (preventing flooding etc) an area of land was reclaimed and a proper lakefront created in the 19th Century. Previously there had been a water course along what is now Bahnhofstrasse (originally part of the city defences) and this was blocked as part of the work. This why that if you look at the buildings, you will find there are a lot of very old (mediaevil) buildings in the side streets of Bahnhofstrasse, but none actually on Bahnhofstrasse. There wasn't a road here in those days but just the city wall and the moat. Also towards the lake you will find that the mediaevil houses stop about where Fraumünster is and beyond that towards the lake it's all 19th Century or newer. This was marshland previously to that. Also, there was no bridge at Bellevue until the earl 20th Century (about) and the first bridge was the one at Fraumünster.

Could be, but I doubt it as the Limmat was dredged at the time that the various islands were removed. The present day Coop at Hauptbahnhof stands on what was once an isalnd as there was once also a water channel between it and the houses. The road underpass you drive through when passing haupbhanhof headed for the Urania bridge is actually part of that former channel. When this was blocked they had to dredge the Limmat to make it deeper to take the extra water.