Republicans did take these races seriously. I'm tired of this ridiculous, revisionist myth that they didn't. It undermines just how strong Tester and Manchin were as incumbents. A lot of people on this site and elsewhere expected them to lose. They didn't, in fact Tester had his best election yet -- first time he won a majority.
And who would be stronger GOP candidates anyway? Both challengers were and still are statewide elected officials. It's not like they just pulled random nobodies off the street and used them as sacrificial lambs. They contested these seats.
The reason the Democrats won is number one it was a good year for Democrats across the board, and number two Tester and Manchin are uniquely strong incumbents for their states. Additionally, Manchin might have been saved by the Kavanaugh vote. Tester was so formidable even that didn't sink him while it might have sunk McCaskill, Donnelly, and Nelson (I think Heitkamp was screwed regardless).
I think you're right on Tester.
Manchin on the other hand might have won anyway (by the skin of his teeth at best) but Jenkins would have done better than Morrissey. Morrissey wasn't a particularly strong candidate.