Governor was never going to be able to appoint a replacement, Kentucky specifically removed Beshear's power of appointment because they saw he was a democrat.

Realistically, there was never going to be an election before November, so in practice the "Mitch must make it past the special election deadline" was about the time between November and January. The special election would have been effective immediately, whereas the normal election is effective in January, even if the special election was on the exact same day the normal election would have been.

Note that Lindsey Graham's death was no big deal because that republican governor had appointment power.