I think Corker would be the strongest candidate, he's the only one who wasn't a NeverTrumper and has some credibility with the base. Plus, it's not impossible he could win or come close, in early 1975 Ford polled way ahead of Reagan but when the campaign got started the gap was closed. At least Corker would stand a slight chance, though would probably still lose.
Corker lost any credibility he had with the "base" the second he defied Trump.
Probably but he'd still lose less badly than the other three and could consolidate many former Rubio and Cruz supporters. Trump wouldn't have been the nominee if it had been a two way race from the beginning. He would likely be the nominee in 2020 but it doesn't have to be 80-20, it can be 60-40 or 55-45.
Anyone managing to get 40% in a primary against incumbent president Trump seems like a reach. Really, the only realistic way I see Trump losing re-nomination is if the conservative media complex turns on him, and says that he's not really a conservative, and instead urge a "real conservative" to take his place. If Limbaugh, Hannity, Fox News, etc. turn on Trump, then yes, he could lose. That is very unlikely, but I guess not completely impossible, because I suppose the Dems could take Congress in 2018 and Trump could decide that "making America great again" now involves cutting a bunch of deals with Schumer and Pelosi.
But if that happens, and Trump becomes that weak because Limbaugh and Hannity have turned on him, then presumably he'd get multiple primary challengers rather than one, such that he could still win re-nomination on a plurality victory (like in 2016).
