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)
This is true. That being said though, I doubt the ballot-stuffing efforts by Republicans downstate were that extensive. After all, the counties in question were ridiculously small and would have been unlikely to net many votes. Plus, I have no doubt in my mind that Nixon, yet unburdened by his election loss that year and in 1962, was a much more honest and fair campaigner than he would later become, definitely more so than Kennedy and his brother were.
Contrast these circumstances in Chicago, where the powerful Democratic machine under the Mayor could have affected thousands of votes in a state where the margin was miniscule. Hell, even Hillary Clinton attests to witnessing voter fraud in Chicago in 1960, back when she was a Republican.
Not that it matters much to be honest because as far as I'm concerned, Nixon won the popular vote due to peculiarities of the election in Alabama.