There wouldn't be any need to raise those Grafs to Duke. The title "Elector" was largely seen as a superior rank to any German title, and so holding the electorship was an automatic promotion above the non-elector nobility of Germany. There would be no need to equality of ranks between the electors either, since as electors they all held equal rank.
Pretty much why the Count Palatine of the Rhine was equal in rank to two electors who were kings, though in reality the situation was more complicated.
Yes. I just thought it looked neater
But I guess the new German Reich would want to maintain the appearance of being the legitimate HRE