There are many election law issues surrounding presidential elections, in which people assume the laws are far more restrictive, than they actually are.
1. In approximately half the states, fusion is legal for president. It is impossible to say exactly how many states, because sometimes the details matter (is it fusion between two qualified parties, or a qualified party and an unqualified party, or between a party and an independent).
2. Sore loser laws do not generally apply to presidential candidates. Presidential candidates who ran in presidential primaries, and then appeared on the general election ballot under a different label, have included Theodore Roosevelt, Robert La Follette, John B. Anderson, Lyndon LaRouche, and David Duke. No state has ever kept a presidential candidate off the general election ballot because he was a “sore loser”, except that Mississippi kept Lyndon LaRouche off as an independent in 1992 on those grounds. He tried to sue, but couldn’t obtain an attorney.
The logical basis for “sore loser” laws does not apply to presidential candidates, because the true candidates in November are candidates for presidential elector. Article Two of the Constitution makes this clear. Presidential candidates’ names appear on November ballots in their capacity as labels for competing slates of electors, not as candidates per se.