Well, now that the Supreme Court has unanimously ruled that Trump is eligible, this entire scenario is nonsense. The only possible way that Democrats could deny Trump the presidency by failing to certify his win, is by engaging in an act of insurrection themselves. Unlike what happened on January 6, 2021, this would be an actual insurrection against the Constitution of the United States, since it is abundantly clear that whether they like Trump or not, he is eligible to be president of the United States, and would, in fact, be so if elected by the people of this country.
Ditto for Joe Biden, or any other Democrat, for that matter. The rules in the Constitution are quite clear, and until the last few years of Democrats not recognizing election results and plainly cheating to “win” wherever and whenever possible, everything went quite smoothly. I would expect that if it is clear that Biden, or any other Democrat won legitimately, that Republicans would be unhappy, but would do the right thing. Likewise, the entire nation expects the Democrats to do the same.
SCOTUS did not “unanimously rule that Trump was eligible.” They unanimously ruled that states could not determine for themselves if federal candidates were ineligible under the 14th amendment, but that it was up to Congress to determine how section 3 could be enforced. They took no position on whether or not Trump was guilty of insurrection or could be disqualified by Congress.