Question: Why didn't Nabokov destroy his manuscript "Laura" himself? Did he finish it, then set it aside months or years before his death? Or did he die in the middle of its composition?

If he died during the middle of its composition, then that would make it an unfinished work—it should be destroyed. No writer wants to see an unfinished work limping out into the world, half-formed.

If, however, he finished the work and then set it aside . . . that's something else. Then it should be published, especially if what Dmitri Nabokov is saying about its importance is true.