Short answers, pulled from the story.
Jacob Singer has been dead since the moment he was stabbed with a bayonet in the Mekong Delta on the 6th of October 1971. The entire narrative depicts his dying process and journey through the intermediate state between life and death before he is finally accepted by military medics in a triage tent.
Bruce Joel Rubin wrote the script for the 1990 film Jacob's Ladder after spending two years in a Tibetan Buddhist monastery in Nepal. Rubin viewed the film as a modern interpretation of the Liberation Through Hearing During the Intermediate State from the Tibetan Book of the Dead.
The film reveals that Jacob Singer and his platoon mates were test subjects for a secret military experiment involving a hallucinogen known as BZ. The drug was designed to increase aggression and caused the soldiers to turn on each other in a homicidal frenzy before they were discharged for psychological reasons.
Adrian Lyne created the visual effects in the 1990 film Jacob's Ladder by filming actors shaking their heads at a low frame rate to create horrifically fast motion when played back. This technique was used to visualize the internal state of a dying mind and to create images resembling thalidomide deformities.
In the final scene of the 1990 film Jacob's Ladder, Jacob is led up a staircase into a bright light by his son Gabe who had died in an accident before the war. This ending signifies a release and a transition to a different state of being rather than a traditional happy ending.