Beats me, but I am sure state ballot access laws allow for replacement if candidate is dead or incapacitated. It would shocking and irresponsible if they didn’t, but it could be in some states. It wouldn’t surprise me.
Yes, but only up to a certain date. In my state I think ballots can't be changed in the 60-day window before an election.
So if the election were Nov 8, then the deadline for changing the name on the ballot would be something like Sept 9th.
Then after Sept 9th, if a candidate became disqualified (by incapacitating illness, death, or being forced out by the party), legally it would be too late to put a replacement name on the ballot. However in NJ the Dems have gotten around this law at least once.
Like everything, "it depends". Remember that John Ashcroft was beaten in the Missouri Senate race in 2000 by Mel Carnahan -- who was dead.
But I don't think Clinton would voluntarily relinquish her position on the ticket. I'd be suspicious that the story is bogus.