I'm trying to think of the ways that the Supreme Court could handle these cases.
They could decide that Trump engaged in an insurrection and prohibit him being on the ballot anywhere.
I don't think that they could do that, as the states set the requirements for who can be on the ballot. For example, Michigan's Supreme Court said that Trump can be on the primary ballot because the Michigan constitution says nothing about whether primary candidates must meet the requirements for office.
Or they could decide that there is insufficient evidence that he engaged in an insurrection and prohibit him from being removed from ballots.
They could
maybe do that - I know the Colorado decision explored that question in detail, and both courts that heard the case concluded that Trump had engaged in insurrection based on the evidence presented. In theory, they could review that evidence and conclude that it is insufficient proof of Trump having engaged in insurrection, which would mean that he couldn't be barred based on the 14th Amendment.
A full federal investigation isn't really necessary, because he's not being convicted of anything. It's a purely civil decision, and the barriers of evidence are lower in civil court. See E. Jean Carroll's defamation case against Trump for an example - the jury found that Trump had
most likely committed the offense of which Carroll accused him, which meant that he was defaming her by calling her a liar. However, he was not convicted of sexual assault, nor was he punished for it. His punishment was purely for his defamation of Carroll.
They could decide that the 14th Amendment doesn't apply to the President, since the President isn't specifically mentioned by it, and prohibit Trump from being removed from ballots. Possible, but one has to consider the intent of the 14th Amendment.
They could make that determination, though I don't think that they could
prohibit Trump from being removed from ballots. Deciding that the 14th Amendment does not apply to the President would just mean that that could no longer be used as justification for excluding Trump from the ballot. States could, at least in theory, still find other ways to exclude him based on state law.
They could decide to just to punt on it and let the rulings of the state courts stand, since it's up to the states to choose electors after all. I think there's a decent possibility for this one.
I'd say it's probably 50/50 between this and making a ruling on the scope of the 14th Amendment. Honestly, I wouldn't put it past the Court to uphold the states' determinations on this one. There's pretty solid reasoning for the President being covered by the 14th.