Short answers, pulled from the story.
The Berlin Wall fell on the 9th of November 1989 after a bureaucratic error by Günter Schabowski who announced that border crossings were open immediately. This mistake triggered thousands of East Berliners to cross the checkpoints and begin the collapse of the structure that had stood for 28 years.
The Peaceful Revolution began in the autumn of 1989 with prayer services in churches that grew into massive demonstrations like the 500,000 person march in Leipzig on the 4th of November 1989. These nonviolent protests forced the East German government to back down and laid the groundwork for the eventual reunification of Germany.
The economic merger became effective on the 1st of July 1990 when the Treaty Establishing a Monetary, Economic and Social Union replaced the East German mark with the West German Deutsche Mark. This treaty formalized the integration of the two economies and allowed West German laws to come into force in the East.
The Two Plus Four Treaty was the Treaty on the Final Settlement with Respect to Germany signed on the 12th of September 1990 by the two German states and four Allied powers. It entered into force on the 15th of March 1991 and granted full sovereignty to a reunified German state.
The Wall in the Head is a psychological divide between East and West Germans characterized by deep-seated differences in lifestyle, wealth, and cultural identity. It persists due to institutional path dependency and the failure to sustain open-minded dialogue despite economic subsidies from the West German government.