That is absurd. Never Trump consists of a few hundred butt hurt conservative journalists and assorted politicians and comment board trolls. The vast majority of the Republican base will vote for whomever the nominee is. That may not be enough to win, but it certainly be enough to be competetive no matter who the nominee is, even if its some candidate to named later.
And the logic that Hillary is going to take the Congress is especially absurd. Even the people who think she is going to win think she will win by default and not because she is popular or a good candidate. Even good candidates often don't have coattails. Moreover, even if Republicans are angry over Trump, why would they then refuse to vote for their Senator or Rep? Most importantly, if you are right that Hillary is going to win in a walk, that makes the Democrats taking the Congress even less likely because people will ticket split to make sure there is some kind of check on what she does.
The pants shitting over this election is just astounding. It really is. It is one thing to think Trump won't win in November. It is quite another thing to then extrapolate that into some kind of apocalypse. Maybe if the Democrats were not as badly split as they are and had anything but a horrible candidate. But with that candidate and being split really even worse than the Republicans? No way.
Source : https://reason.com/blog/2016/04/21/am-links-gop-insiders-warm-to-trump-bern