Question about premoves

Say it's the opponent's move, I have mate in one, and they just sit there letting their clock run down. If I premove the mating move, and close the game for whatever reason, will it still make the move if my opponent actually moves a piece with like a second left or whatever annoying shit sore losers do sometimes?

I had a game today where I got disconnected in such a situation, but hadn't premoved, and I'm wondering if doing so would have salvaged the win for me.

No. AFAIK, premoves are not sent to the server, but only executed on your computer. If you close the game, your premove will be deleted.