One possibility I see is that the SC tries to retain some credibility by punting this back to the states. They rule something along the lines of, without explicit guidance from Congress then each state may come up with their own rules for determining how the insurrection clause applies, and these rules will hold until such time that Congress speaks up, even if they are inconsistent with or even outright contradict the rules from another state.
Thus he technically loses, and is stricken from both democratic Colorado and Maine, but no one will be able to use the SC ruling to get him off the ballot in e.g. Texas or Alabama.
The other way the SC could punt is simply to run out the clock, and when the GOP primaries have been decided simply declare the issue moot. (This wouldn't work if the guy ends up winning the Presidency as then they'd have to resolve the question of his ineligibility at some point - but if he loses in the end they can just wait for him to lose and then say it's moot, because deciding the answer wouldn't have changed the outcome - he wouldn't have become President again either way.) The cynic in me can see the SC preferring to punt this way as it leaves the door open to actually ruling in favor of using the insurrection clause this way - in some future election cycle against a Dem presidential candidate who doesn't deserve it.