Catholic books say that even ordinary jurisdiction can be automatically supplied by the Church.
So, not having a bishop of a diocese to follow is not strictly necessary.
And, when it comes to electing a pope, Catholic books say that this is not an act of jurisdiction. Historically, clergy merely had to be citizens around Rome. An election is ASKING a man to be their bishop, not appointing him against his will.
This means, that if all clergy in Italy died, and some traditional priests traveled there and became citizens, they could elect a bishop from anywhere around the world to be their bishop - the Bishop of Rome. Accepting it, he would then have full ordinary jurisdiction over the Catholics of the whole world.