Were there some fraudulent votes cast? Sure. Millions? Not even close.
Agreed. I would venture that almost no elections are decided by voter fraud, with the only realistic candidates being those close enough to trigger recounts.
No modern elections, probably (although a case could definitely be made for 1960, particularly in Illinois). Go back before 1900 however and fraud allegations were so rampant, that in some cases certain states had their EVs discounted.
In Illinois 1960 it's impossible to know who really won. Rs were doing voter fraud in Downstate and in the collar counties, and Dems were doing it in Cook. (I think)