Marcos was in very poor physical (mentally sharp as usual) health at the time, he was dying of lupus. Likely the strains of office cause him to die by 1988, one year earlier than OTL. By that time, there would be a power struggle between Enrile and Imelda, which Enrile would easily win with the military's backing. Also, a council led by Imelda, PM Cesar Virata and Gen. Ver took care of much of his duties at the time.
Re assassination: Marcos might not have ordered it, but he definitely gave a yellow light. The military would not be insubordinate if he gave them a red light, as demonstrated when he refused free-fire orders on national TV IOTL. He warned Ninoy through Imelda that he would not allow him to return back in 1980. Remember, this was a man who allowed death squads to roam the provinces at will to assassinate his opponents and who bombed the Liberal midterm rally (the suspect was a disparido Constabulary officer) in 1971, causing a midterm wipeout in November.
Re election: Marcos might even win, because even according to NAMFREL, Cory beat him by only 800,000 votes, less than half a percentage point.