I think he could stay on until 1992, enough for the Democrats to hold up his successor like the conservatives did to Warren in 1968.
Absolutely decisive. 5-4 conservative becomes 5-4 liberal. But Souter, Kennedy, and especially O'Connor will be more conservative. Depending on who replaces White and Blackmun, the court will still not be as liberal as the Warren Court. Souter was more of a conservative in the mold of Harlan, and won't be keen to legislate from the bench.
Marshall had wanted to retire for some time, but obviously didn't want to do it during the first two administrations to veto civil rights bills since Andrew Johnson. He joked to his aides, "If I die, just prop me up! If he and Brennan knew that Bush would only have one term, I'm pretty sure they would have tried to stay on. But they probably looked back on the last time a party broke two terms, to FDR-Truman, and assumed Bush would win 2 terms. Bush would have to become less popular early on in his presidency. He was in the '60s and '70s when Brennan went in 1990, and Marshall left in 1991 after the Gulf War, when Bush was staggeringly popular.
So I do think you have to avert the Gulf War to have Marshall stay on. Brennan is harder, but since he was replaced by Souter, a lot of people don't care as much. Pretty much everyone at the time realized how atrociously unqualified Clarence Thomas was.