a.) The Governor of Kentucky, Andy Beshear, is a Democrat, and will most likely pick a Democrat to replace McConnell (I say most likely because Beshear has been known to support bipartisanship and might take that as far as to appoint a Republican to the Senate). Replacing a Republican with a Democrat would give the Democrats 51 seats, and they'd no longer require Manchin's approval on issues like the Nuclear Option To End The Filibuster.
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UPDATE: Ignore what I wrote about McConnell being replaced by a Democrat; there is a law in Kentucky prohibiting the replacement for a senator being from a different party, so McConnell's replacement would be a Democrat.
Even ignoring the law there would be an exactly 0% chance of Beshear appointing a Republican. He's not a Manchin or Bel Edwards type. He's a pretty standard liberal Democrat who would never go nearly that far in the name of "bipartisanship."