I don't know what law they would invoke to reject him after the election.
And in the end, would SCOTUS have the spine to support a state in rejecting an elected candidate, even if that candidate was a recently convicted criminal?
I'm afraid your SCOTUS is a busted flush. The right hand side of it applies strict constructionist opinions when it suits their politics, and makes sh-t up when it suits them.
In this case, there is no text to say congress is the only body capable of enforcing 14/3. So they had no reason to justify an opinion making that ruling in the plain text of the constitution. And if they were also consistent with their usual states' rights principles, the justices on the right should have left Colorado's decision to the state.
It's just partisan politics all the way down. And they won't live it down. SCOTUS is not a trustworthy decision-making body, regardless of the figleaf of unanimity. Three justices plainly dissented, but surrendered to the appearances because it was an electoral issue. In my view, this was far worse than Bush-Gore, where they couldn't avoid a hard choice. Here, they had the choice not to get involved. But they chose to. They have made 14/3 as useless as the whole impeachment process, offering no protection against bad actors. And their reputation as a non-partisan body is tarnished as a result.
I've said it before. The US lacks a non-partisan institution to adjudicate trust-related questions in American politics such as impeachment and fitness for office. McConnell's cheating over the composition of the court is a disaster for SCOTUS. Dems know that they will decide political issues on partisan lines when it counts. A new body might sensibly be made up of folks like ex-presidents and some others. Obviously, if an ex-president is the subject of the decision, they don't get to be a decision-maker.
ps That's where a constitutional monarch fits in our system. Someone above the political fray. Such a body would mimick that role and relieve your courts of the responsibility of inventing things it has no business inventing to avoid addressing the issue of whether Trump, a known former insurrectionist, should be allowed to run for president. Like Hugo Chavez in Venezuela.