Here is where I think the SH*T could REALLY hit the fan:
Neither Obama nor Romney get the necessary 270 electoral votes. It is thrown into the House of Representatives for resolution. Obama turns out has led in the national popular vote. (The media hypes this point). Nonetheless, The House being majority-controlled GOP, it awards the Office of Presidency to Romney and certifies him as such in Congress Assembled.
The hype in Obama's core community has nothing to do with rules nor the order of Law nor in the precepts and procedures in the The United States Constitution (which they won't education people as they start to smash store windows and pull truck drivers from vehicles to beat the snot out of them.). The only rallying cry will be "Obama won the majority! Obama won the votes! But the WHITE House of Republicans took it away from him (us). The Crackers have denied us a president again!" It would be quite a potent and nascent battle cry across the land. I would say, double up on the sandbags and citizen patrols checking entry into safe neighborhoods. Course, the FBI, Justice Department and federalized National Guard would consider that "unconstitutional, racist and unwarranted".
What a perfect storm, this Romney wins the House vote for President, with an Obama majority of the national total popular vote. Yikes.
The electors assemble in their respective states, and send certified ballots to Congress. Those ballots name an individual person for the office of President, and another individual person for the office of VP. I think you are correct on the timing, the first week in December. But I'm going from memory on watching Bush v. Gore, which pushed tight against the statutory deadlines.
Congress opens the ballots and counts them, state by state, IIRC, in alphabetical order. This is the point in time when challenges can be made, e.g., ineligible candidate or improper ballot from a state. John Kerry challenged Ohio's ballots, but the Congress voted the challenge down.