God forbid that any of the current candidates should meet with misfortune , but what happens if...
A. The President-elect dies (or resigns) between the electoral college vote and inauguration
"If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President." That's the twentieth amendment, section 3.
B. The President-designate dies between the popular vote and the electoral college vote
Unclear. As you pointed out, the 1872 election is our only guide. From that, we can surmise that the electors will be free to vote for anyone they want (but then, of course, they're already free to vote for anyone they want
C. A party Presidential candidate dies between the convention nomination and the popular election?
Again, that's unclear. Different states have different laws. This article
answers -- or doesn't answer -- a lot of these questions.
Also, the same questions if the Vice-President elect/designate/candidate dies.
A disaster of biblical proportions. Dogs and cats living together. Mass hysteria.
Didn't that actually happen once?
Yes, but no one noticed.