SCOTUS will rule x number of ballots invalid (whether these are un-supervised, late, cured, who knows) in various states (probably PA, MI, AZ, GA) and then we'll see how the numbers fall re:states and popular vote
and/or
It's impossible to exactly identify which ballots are affect, just the raw number, and so states will 2a) use their own legislature to send electors, weighing the court rulings into their decision, in which case popular vote is meaningless (which it is anyway) or states will 2b) send two sets of competing electors because it turns into an inconclusive cluster fuck
and/or