Short answers, pulled from the story.
The Emperor arrived at Edo Castle on the 26th of November 1868 and renamed it Tokyo Imperial Palace. He returned to the city on the 9th of May 1869 when the name officially changed to Tokyo.
A fire consumed the Nishinomaru Palace on the night of the 5th of May 1873 after Shogun Tokugawa Yoshinobu vacated the premises. The new imperial palace was subsequently constructed on this site in 1888.
Architect Junzō Yoshimura designed the new palace complex that spans 24,175 square meters across seven wings. The Kyūden was completed in 1968 and put into use starting April 1969 with materials sourced domestically within Japan.
Main palace grounds remain generally closed except for reserved guided tours operating from Tuesdays to Saturdays. Public access is granted through the Nakamon inner gate on January 2 and February 23 while parts of Fukiage Garden sometimes open to general visitors.
The East Gardens encompass former Honmaru and Ninomaru areas covering a total area designated as Special Historic Relic since the 30th of May 1963. This designation protects symbolic trees representing each prefecture planted in the northwestern corner of Ninomaru enceinte.