That would be the first hurdle. Next, they'd have to either get *all* of the Trump electors to become 'faithless' and vote for their choice, or enough of them to throw the election to the House & Senate.
If that happened, though, they'd likely still control both the House and the Senate, so they'd effectively choose the POTUS and VPOTUS.
While the establishment would probably like Ryan in the top spot, I think Pence would be the 'consensus' choice that would garner enough votes from the Trump electors. He's perfect for them, in that he's malleable and controllable. So Ryan would effectively be his 'Cheney'.
Even if all the stars aligned and they somehow convinced Trump to drop out 'voluntarily', I don't think they could flip more than one of the toss-up states, perhaps two at the most. I could see Iowa, maybe Wisconsin (if Ryan was announced for the 'replacement' ticket beforehand). But they need *all* of the toss-up states - and that is not going to happen at this point.
Of all the times I've considered the EC an anachronism, I'm glad for it now, and that it favors us.