On basis would they refuse to seat Moore? On mere allegations launched on the eve of an election?
Yes. It's not hard to convince people to do something they want to do anyway. They will say he tarnishes the reputation of the Senate and then they would vote him out.
You will get all the Democrats, and at least half of the Republicans to go along with this. It doesn't matter if it isn't fair, they are Senators who can vote as they please.
Should he win, the senate must accept Alabama’s choice for senator. There’s no provision to do otherwise.
It's not going to work like that. McConnell doesn't want him, and neither do the rest of the swamp creatures, like McCain, Graham, and so forth.