“In 2000 Al gore (then vice President) could have just chosen himself as President?”
If Florida had submitted two opposing set of electors in 2000 (they didn’t) and Gore only recognized the Democrat slate submitted (and ignored the Republican slate), members of both houses of Congress could then either accept or reject that (Democrat) slate (by each house voting separately.) If both houses didn’t agree to reject it, Gore technically could have counted Florida for himself, giving himself a majority of the Electoral College and thereby the presidency.