Since the 12th amendment did away with separate voting for President and Vice President, and the 20th amendment contains language for what should happen if either the President-Elect, Vice President-Elect, or both, should "fail to qualify," I don't see how your scenario might result in one, but not the other, failing to qualify. Yet the language is there in the Constitution for it to happen.
My larger concern is about the nature of the parties. When an ambiguity like this exists in the Constitution and the law, it is the nature of the Democrats to push it as hard as possible to drive the "untested waters" to the outcome most desirable to them.
When Republicans are in the same situation, they talk themselves out of doing anything for fear of the "worst case scenario," or they make the most burdensome arguments as reason for not testing the waters themselves.
This is why the Democrats always win and Republicans always lose. Democrats always put the ball in play to see what happens in an uncertain situation, and Republicans refuse to play because they prematurely convinced themselves that they will lose.
-PJ
I understand your point about the differences between the ways Democrats and Republicans approach these situations. I don’t necessarily think the GOP operates the way they do out of fear or timidity. They just tend to be wiser about many things and understand the broader implications of pushing the limits in government. As a wise man once said: “Don’t take down a fence unless you understand why it was put there in the first place,”