There is a catch here. Although the postal ballots were overwhelmingly favoring Joe Biden, from what I have heard, the late arriving postal ballots were more favorable towards Donald Trump. At least in states such as Arizona, Trump held a considerable margin with these late arriving ballots. So I don't think that it is going to make any difference in the outcome. Anyway, the total number of these ballots may not exceed a couple of thousands max, per state.
Was Trump ahead at midnight or not?

The election itself was completed, "consummated", on the 3rd. Counting all the votes was not, and never is but that case had nothing to do with counting the votes but that the voting was finished on that day. You're grasping at straws.
The fact that mailed votes came in for days after the election, and were stripped of their postmark, and mixed with other ballots, shows that we can only us that which was already counted by midnight.
