How to go about answering the most interesting part of this question: what would it take for the NPVIC to actually reach majority?

Currently, Republican states don't have the incentive because they benefit from the Electoral College. If this changes (unlikely?), it tests the commitment of the Blue states.

That leaves the swing states. They benefit a lot from being swing states: it gets them all kinds of favors from presidential candidates. In addition, the NPVIC might be hard to pass in split legislatures.

So the swing states have to become Blue states, which would mean decisive victory of Democrats over Republicans, which seems unlikely - maybe it might happen if the Republican party implodes? But in that case, the NPVIC is irrelevant.

And when it becomes relevant again, there's strong incentives in swing states to change the rules again, I suppose?