What if the Electoral College result at the meeting date differs from the final?

Technically, there are 50 answers, but the basic answer is that the Electoral College meeting happens when it is scheduled to happen by federal law, and if some number of votes have not been counted in some state, those votes do not influence which electors are selected for the state. This is true whether this is a red state, blue state, or red-blue mixed state. Voting laws do not mandate ridiculous measures to ascertain that every vote has been counted correctly, and it is particularly common for write-in votes to not be counted unless there is good evidence that it might matter (you'd have to look at the laws of a particular state to see that that means).

The crucial law in Washington is RCW 29a.60.190, which directs that

twenty-one days after a general election, the county canvassing board shall complete the canvass and certify the results

provided the ballot is postmarked 8:00pm on the day of the election and is received no later than the day before certification. That defines the cut-off point, and any ballots received afterwards simply are not considered. It is a crime to not certify results, if the results can be ascertained with reasonable certainty. Subsequent sections say what that means, and still insists that the canvassing (vote counting) must be completed by the deadline. RCW 29a.56.340 then says that electors shall meet at noon on the day set by federal law. In other words, the law simply does not allow the sec'y of state to put off certifying the results of an election.

At the federal level, electors are appointed on election day. If a state has "failed to make a choice" by the deadline (which is the first Monday after the second Wednesday in December next following their appointment) the state legislature of such State can take matters into their own hands – this is why Washington law removes all wiggle room. 3 USC 5 in particular sets down a deadline for resolving controversies. Thus in Bush v. Gore, 531 U.S. 98 because the Florida vote-count controversy could not be resolved by a certain procedure applied state-wide while still satisfying the federal statutory deadline, the vote count that was in hand on that day was the final vote count.