Short answers, pulled from the story.
The French government led by Adolphe Thiers made the decision to return Napoleon's remains from Saint Helena. A twelve-member committee formed to decide on the location and outline of the new funerary monument.
Napoleon's remains were transferred into the sarcophagus on the 2nd of April 1861. They had lain in a nearby chapel since 1840 while construction progressed around them.
The massive sarcophagus is made of purple Shoksha quartzite mined in Russian Karelia. This stone arrived in Paris on the 10th of January 1849 after being quarried in 1848 by Italian engineer Giovanni Bujatti.
Louis Visconti emerged as the selected architect after a protracted selection process that concluded in 1842. He finalized his design around mid-1843 creating a circular hollow beneath the dome of Les Invalides.
Adolf Hitler ordered the transfer of Napoleon II's coffin from Vienna to be placed next to his father on the 15th of December 1940. The decision came following advice from his ambassador to France Otto Abetz.