People can talk about "declaring" all sorts of things, but unless you get both Congress and the Supreme Court to go along -- which ain't happening -- it's delusional.
There was a fairly recent Texas Supreme Court case, Pressley v. Casar, involving a challenge to an Austin city council election. The challenger lost in the trial court and the court of appeals, and appealed to the Texas Supreme Court. But while the case was pending in the Texas Supreme Court, the winner was re-elected and his new term had already begun. The Texas Supreme Court dismissed the case as moot, because a challenge to an official’s election becomes moot when the official’s term expires.
It is hard to see how the exact same principle would not apply to a challenge of the appointment of a presidential elector after he has voted. The only way around it is to ignore, as many do, the distinction between a state’s appointment of electors and the actual presidential election. It is clear that Trump’s own lawyers understood, though, that their challenges would become moot some time between the “safe harbor” date and January 6.