The worst case is that he uses his power to prevent a decisive outcome against him. If Trump sheds all restraint, and if his Republican allies play the parts he assigns them, he could obstruct the emergence of a legally unambiguous victory for Biden in the Electoral College and then in Congress.
The Electoral College system doesn't have ambiguities in the way this requires; unlike, say, a popular vote system, it's specifically designed to eliminate ambiguities. For the President, the process is like this. The states certify their election votes. The Electors are chosen in accordance with state law. They vote on the day decided by Congress, currently the Monday after the second Wednesday in December. Under the Electoral Count Act, they have to complete six Certificates of Vote, signed by all Electors; one of those six goes to the office of the President of the Senate. Much of Gellman's looney-tunes scenario requires there to be rival Electoral College ballots (for multiple states!); of course, the only actual Electoral College votes are those which are consistent with state law. Klain's action of booking a rival room in 2000 was purely a just-in-case move; litigation over whether the votes had been properly accounted was still going on, the Republicans booked a room in case the litigation favored them, the Democrats responded by booking a room as well, in case it favored them. The only way this could have become an issue is if (1) the litigation did not end by the day the Electoral College was supposed to meet despite everyone having an incentive to make sure it did; (2) the litigation did not settle who had the legal claim to the Electoral College votes by the time the vote was counted, despite everyone having an incentive to make sure it did; (3) Congress could not come to an agreement about which to reject.
Congress presided over by the President of the Senate counts the votes; if members of Congress question the legitimacy of any vote, there is a legal process for doing so; no state's Certificate of Vote can be rejected without a vote of both chambers of Congress (such a rejection has happened in only two elections). If a candidate gets sufficient Electoral College votes, he wins, unambiguously. If no candidate gets sufficient Electoral College votes (which, despite Gellman's imaginative attempt to suggest otherwise, logically includes the case where it is close but Congress does not know what enough of the Electoral College votes are, like the rival ballot case), the House voting by state delegation chooses one of the three candidates with the most Electoral College votes. They can choose any of the three they want; all it takes is 26 state-delegation votes. (Currently Republicans hold more state delegations in the House, but it would be the new Congress that mattered.) Whoever gets those wins, unambiguously. If nobody gets enough, they keep balloting until someone does. Much the same process is done for the vice president, except it is the Senate, not the House, that decides the matter in case no vice-presidential candidate gets enough Electoral College votes. If the House can't pick a president by Inauguration, whoever is elected vice president becomes the president. If the House can't pick a president and there is no vice president by Inauguration, the line of succession kicks in and the Speaker of the House becomes president. There is only a limited set of options for President -- Trump, Biden, Pence, Harris, and Pelosi, very roughly in that order of likelihood. No matter what happens, it will be unambiguous. Even in the worst case scenario, with both the College and Congress deadlocked, we would still have an unambiguous winner as Nancy Pelosi is sworn in as president.
I don't say that it's absolutely foolproof; I mean, politicians can be such fools that there is no such thing as foolproof when they are involved. But the process is, again, specifically designed to cut out any potential for ambiguity at each stage.
And this is all, of course, without even considering the fact that Gellman's scenarios require obviously implausible events like Pence deliberately burning his political career by going rogue in order to guarantee that Trump is president (and what is more, that Trump rather than himself becomes president) and Congress doing nothing to stop it.
It would be foolish to deny that history can throw a curveball on occasion for which no one can adequately plan, but throughout Gellman's article we are not so much dealing with possibilities as with shadows of shadows of possibilities. Yes, it is true that if the entire world inexplicably goes mad all at once, the Constitution has no mechanism that can stop it; but that is not only not surprising, it is not the kind of possibility worth worrying about. It would be like planning your vacation while worrying about the fact that you haven't figured out what to do if you get hit by an asteroid while on the beach.