This may not answer all of your questions, but some. Internment was executed via an executive order from the president, and ended through the supreme court:

President Roosevelt signed Executive Order 9066, authorizing the War Relocation Authority to force 110,000 Japanese and their American-born children into relocation camps. Internees relinquished their communities, homes, and livelihoods for cramped barracks in isolated interior areas of Arizona, Utah, California, Wyoming, Arkansas, Idaho, and Colorado. Officially, the government declared that the forced relocation was necessary for Japanese Americans’ safety.

Regarding legal challenges:

Constitutionality of the internment were challenged in the courts as early as 1942 in the cases of Hirabayashi v. United States (1943), Yasui v. United States (1943), Korematsu v. United States (1944), and Ex parte Endo (1944). Hirabayashi and Yasui’s convictions for curfew violations were upheld on appeal by the Supreme Court. Korematsu’s conviction for violating exclusion laws was also upheld by the Supreme Court. In 1944, Endo’s challenge to the internment order and won, opening the way for the end of the incarceration.

Regarding the end of internment:

December 18, 1944, the Supreme Court ruled unanimously that Endo "should be given her liberty" and released from custody, since her loyalty was clearly established.

In the ruling, Justice Murphy stated: "…detention in Relocation Centers of persons of Japanese ancestry regardless of loyalty is not only unauthorized y Congress or the Executive, but it is another example of the unconstitutional resort to racism inherent in the entire evacuation program."

Major Pratt, commander of Military Area #1 at the time, ordered the suspension of the exclusion orders; and detained Japanese Americans were now free to return to their homes on the West Coast.

On a side note here. Arent those just concentrationcamps?