Short answers, pulled from the story.
The partially constructed Romanesque church was solemnly dedicated to Saint Stephen on the 12th of May 1147. Conrad III of Germany and other German nobles attended this ceremony before embarking on the Second Crusade.
Duke Rudolf IV laid the cornerstone for a westward Gothic extension on the 7th of April 1359. His project aimed to increase the religious clout of Vienna beyond its status as a mere parish church.
On the 12th of April 1945, civilian looters lit fires that reached the cathedral and caused the roof to collapse. Wehrmacht Captain Gerhard Klinkicht disregarded orders from city commandant Sepp Dietrich to fire shells and reduce the building to rubble.
This new bell weighs 20,130 kilograms and stands as the largest swinging bell in Europe. It entered service in 1957 after being cast using metal salvaged from the original Old Pummerin which crashed to the floor during the fire of 1945.
Full reopening occurred on the 23rd of April 1952 when the structure finally stood ready for public worship again. A limited reopening had taken place earlier on the 12th of December 1948 following immediate post-war reconstruction efforts.