“””Let’s define “ineligible” voters. In order to vote in a national election, you must be a citizen of legal age, of sound mind, and have no felony conviction, right? Please help me out if there are any other restrictions I’ve forgotten about.”””
You overlooked an extremely important restriction re national elections.
Namely, you must prove your residency in the state prior to voting.
It seems there are hundreds of thousands of folks who are registered in two or more states.
Since the President is chosen based upon the majority of electoral votes and not popular vote, we need to close the sewer where folks pick and choose which state they vote in.
That's true, but by requiring voting in person, combined with a requirement to see a photo ID, much of the cross-state voter fraud is eliminated.