Concerning the pardon & 3.5 years house arrest. Do a Google search for William Calley. If you read his bio you’ll see the public outpour of support was the reason for the pardon. After his conviction state governors across the states protested & pulled publicity stunts. The WH was bombarded with mail, phone calls & telegraph for weeks. Even then Governor Jimmy Carter of Georgia told everyone in Georgia to drive with their headlights on during the day to show support for Calley. Just like like with Gallagher, the POTUS then gave a pardon because of public relations.
If you read the bio you’ll also find that William Calley was perhaps the dumbest & most junior officer to take part in the massacre. He was a H.S dropout who was describe as a guy who had no common sense, who couldn’t read a map or command men to save his life. He was an officer but supposedly given something similar to a sock party by his men. Another pansy for the real soldiers who went to West Point & other important military academies where something like a massacre would tarnish the image of the institution.