I stand corrected. Yes, technically they could vote for someone outside the group of selected candidates on the ballot, but that is so rare, there have only been 7 electors to do this in all of the elections in the last 100 years. I'm thinking they would follow tradition and pick from the "approved" group remaining on the ballot - unless die-hard democrat electors tried to select another democrat that wasn't on the ballot...and it's almost certain they would not be able to consolidate enough votes for any one person, even if Obama "nominated" someone, LOL. They'd split it between Hillary and Biden almost for sure, regardless of what the Obamameister said. And, they'd all be divided off in their individual states, unable to truly negotiate a settlement candidate. Sweet. It's a mess, but I hope it happens, just so I can watch the MSM choke on it after their criminal conspiracy to manipulate the election. Justice, methinks.
Yes they might split their votes, but I really expect none of the votes for President to be for Biden, if he had that much support, he'd have been a viable candidate himself, which he clearly was not. Hillary was, and few others. Possibly some one who has a significant support, but chose not to run against Hillary.
But regardless, as long as one elector votes for someone other than The One, "just in case" rather than the more usual "protest" vote, then the House could pick that person, or from those persons who got "rogue" votes. If there was one Democrat among the group of eligible people who got electoral votes, the House would vote that person in so fast it will make our head spin. Not a good outcome, but a Constitutional one.