Short answers, pulled from the story.
The Berlin Declaration of the 5th of June 1945 was a document signed by the four Allied powers, the United States, the Soviet Union, the United Kingdom, and France, assuming supreme civil and military authority over Germany after its military defeat. It went beyond the German Instrument of Surrender of the 8th of May 1945, which covered only the military capitulation, by establishing a formal legal basis for Allied governance of the German state.
The Berlin Declaration was signed by Georgy Zhukov for the Soviet Union, Dwight D. Eisenhower for the United States, Bernard Montgomery for the United Kingdom, and Jean de Lattre de Tassigny for France. Each commander signed four copies in English, French, Russian, and German.
The German Instrument of Surrender of the 8th of May 1945 covered only the military capitulation of German armed forces; it made no provision for the surrender of the German civil government. Because the Allies rejected Karl Dönitz's claim to have established a legitimate successor government at Flensburg, there was no recognised German authority to transfer civil power, so the Allies assumed that authority unilaterally through the Berlin Declaration.
Zhukov refused to sign because Article 10 required Soviet forces to arrest Japanese nationals found in Germany, but the Soviet Union was not yet at war with Japan. Eisenhower ordered the clause removed, and after Zhukov sought approval from Moscow, the four commanders proceeded to sign.
Article 13 gave the Allied Powers almost unrestricted authority to reshape German civil, economic, and legal structures within their zones of occupation. It was applied to carry out de-Nazification of public institutions and businesses, to extract reparations, and in the Soviet zone to redistribute expropriated rural land from large prewar estates to tenant occupiers and expellee farmers.
Yes. In 2005, the European Court of Human Rights found in favour of the legality of the Allied occupation in a case brought by descendants of landowners whose estates had been expropriated under Soviet-instigated East German land reform. The court ruled that the postwar occupation of Germany had been an occupation "sui generis" which had "conferred powers of sovereignty" on the Allied Powers.