Both that and the Eagleton rule show that the parties decide who their candidates are. Not you, despite your schoolyard-echoing claims.
They decide that at the conventions and before ballot deadlines. They do not get to decide who their candidates are in late October. Unless they can rely on partisan judges, of course.
Cite for the No Backsies Law? 
Whatever a state ballot deadline is, that’s the deadline for deciding who your candidate is in that state. Some will be set in stone within weeks of the conventions. Others might take a bit longer. Pretty much no state will allow a ballot change in October absent extraordinary circumstances. And no, “We’re losing!” isn’t extraordinary circumstances.
So, no. I didn’t think so.
True. The last thing Republicans can claim in 2016 is that they have any expertise in picking candidates. They should be asking the Democrats for advice not offering it.
40-56 approval says you guys aren’t any better:
Every time you wave a munch of meaningless numbers, I hear Donald shout, “I’m building a wall!!!”
And Obama going to pay for it! (He’s upped his game…)